Learning transfer refers to the degree to which an individual applies previously learned knowledge and skills to new situations. - source
I already know that the phrase "learn knowledge" is grammatically correct, but it is not commonly used in English because the verb "learn" already implies the acquisition of knowledge. Therefore, it is more natural to simply say "learn" or "acquire knowledge."
But what makes the passive form previously learned knowledge acceptable?
The phrase "learned knowledge" is a red herring here: the focus should be on the whole phrase "previously learned knowledge and skills".
The sentence could use the word "acquired" in place of "learned", but the context is an essay talking about "learning transfer"; because the essay - and, it appears, the site as a whole - is dedicated to learning, it's natural to prefer "learned" and other forms of "learn" over (functional) synonyms. The sentence could also have used "previously-learned", further emphasizing that learning transfer is all about "stuff that was learned in the past".
Using "previously learned" also emphasizes that learning transfer isn't about gaining new knowledge or skills per se, but is about applying the knowledge and skills that the person already has to some new situation. Taking one of the early examples from the linked source: a technician who knows how to replace one particular model of video card can reasonably be expected to replace a similar video card of a different model without further training - applying the "previously learned knowledge and skills" about how to safely replace a Brand X video card to the task of replacing a Brand Y card. The technician hasn't really learned anything new, but has done something for which they weren't explicitly trained.
That same technician wouldn't be engaging in learning transfer if they were replacing a handful of Brand X video cards in a training lab (they'd be learning but not engaged in learning transfer: they're specifically working on gaining a new skill, not applying an existing skill to a new situation). Using "previously learned" highlights that difference.
That same technician, again, would be engaging in learning transfer if they took the knowledge they have about replacing video cards and applied it to replacing RAM sticks, drives, etc. - knowing how to minimize the risk of injuring themselves on the computer case and the risk of static shock damaging components in the computer.
"Previously learned knowledge and skills" also means that learning transfer is not occurring when someone is exploring a whole new area - I'm a fairly competent touch-typist, but knowing how to press down on the right computer keyboard key won't help me learn how to play the piano even though it's sill "just pressing the right key at the right time" (or, at least, the extent to which it will help is negligible).
Thus, there are two key elements of learning transfer:
As you say, it's redundant to use the noun "knowledge" as the object of the verb "learn", so English speakers tend to avoid that collocation. I'd therefore argue that the example sentence demonstrates poor word choice. This chart (despite the limitations of Google Books ngrams) suggests that writers prefer to use other verbs with "knowledge":
When we add "the" in front of each phrase to focus on the verb as a past participle (not past tense) form, "learned" still trails far behind "acquired" (though it seems to have gained a bit of currency within the last few decades):
"learn knowledge" (present tense) is not natural because "learn" is already primarily associated with "knowledge" as concepts, theories, and facts, making "learn knowledge" redundant. We say "learn history" or "learn physics" instead.
"previously learned knowledge" is natural because it is a phrase describing REFLECTIVE activity (past tense) emphasizing what has already been POSSESSED as a result of the learning.
Why not say "acquired" / "gained" / "gathered"? "learned" is more suitable especially in the case of non-theoretical knowledge, as what one POSSESSES after learning practical skills such as driving a car, doing carpentry, playing piano, etc. If one wants to emphasize HOW this non-theoretical knowledge is obtained, i.e. through applying, practicing, and doing, which is the topic of your example, saying "learned knowledge" is better because of a tight link to the EFFORT.
"learned knowledge" is acceptable in this sentence because of the desired semantics: we want to emphasize the EFFORT that the individual has already made in situation A. The purpose of that sentence is to say that just because the individual has done the EFFORT at situation A, it's not guaranteed that the individual can apply that EFFORT to situation B.
How do we call that EFFORT to distinguish it from what has been POSSESSED? Answer: "Learned knowledge and skills". Another answer: "Acquired knowledge and skills". The possession is "knowledge and skills".
To prove this, we can rewrite the sentence by avoiding saying "learned knowledge", although the result is less concise:
Learning transfer refers to the degree to which an individual applies knowledge and skills [the possessed] which have been learned previously [the effort] to new situations.
Or we can even rewrite the sentence using the "active form" of "learn knowledge" to demonstrate that it CAN be idiomatic:
After an individual has learned knowledge and skills in one situation, can the individual apply them to new situations? Not necessarily. It depends on the learning transfer rate.
CONCLUSION: This is not about passive "learned knowledge" being acceptable but active "learn knowledge" is not. This is to achieve the desired semantics: the writer choosing a grammatical construct that can emphasize the effort as well as writing the sentence more efficiently.
Knowledge is usually associated with concepts, theories, and facts which we learn in classrooms and in books, such as Physics, Philosophy, History, etc. Since most people associate "learning" with this type of knowledge, that's why "learn knowledge" sounds redundant.
But there is also another type of learning where the "knowledge" is "know-how" / "skills", focusing on how to solve problems, like plumbing, driving a car, swimming, playing the piano, composing music, writing computer software, and other countless subject of "How to X" books, where theory is subservient to its practical application. These skills must be learned (!), and our language testifies to this: we learn swimming, we learn playing the piano, we learn to drive a car. "Practice makes perfect", they say.
And situations matter, you cannot call yourself a proficient driver if you have never driven outside suburban residential areas, or a proficient pianist if you never use the sustain pedal (because you only play pre-Romantic music), or a well-rounded software developer if you can only write an ASP.NET WebForms program in Visual Basic. An individual who learns well (no doubt guided by a good teacher) can transfer his/her "knowledge and skills" to more situations: driving in new cities that are a LOT more crowded or driving through intricate 75mph California freeways, playing Chopin (not just Bach), and applying previous coding skill to new languages/software-frameworks.
So all 3 elements (theory, skill, and situation) feature prominently in learning, where it's not just ingesting theories, but also applying theories to your body (lots of "know-how" involves training your muscles & hand-eye coordination) and to new situations ("know-where", if I may coin a new term). In the 2 non-theoretical elements, there is knowledge too (!), but of a different kind. In the writer's terminology, those individuals would have high "learning transfer rate" due to their superior way of learning (kudos to their mentors / teachers).
CONCLUSION In light of the above, if we use "learned knowledge" to refer to the "know-how" and "know-where" knowledge gained in the 2 non-theoretical elements above, then the usage should NOT be redundant because "learned" refer to the EFFORT of "practice makes perfect" in various situations.
The idiomatic usage itself IS rare, of which the example analyzed above is one. People usually say "I learn piano. I learn plumbing." Only when they REFLECT on what exactly they POSSESS after the learning EFFORT, do they say they "learned knowledge & skills".
Because it's a sentence describing reflection activity, that is probably why we never see "learn knowledge" in the present tense, but always "learned knowledge" in the past / perfect tense. So I do agree with you that "learn knowledge" (present tense) is NOT idiomatic.
I think using ngram to support the idea that "learn knowledge" is very limited is misguided because the search fails to be restricted to REFLECTIVE & idiomatic usage, i.e. when stressing the 2nd and 3rd type of knowledge.
Your other ngram search is better, since "previously learned knowledge" is REFLECTIVE, but it is still diluted with instances of talking about knowledge of the 1st type, maybe the majority of type implied when people talk about acquiring/gaining knowledge.
Yes, I agree that even for REFLECTIVE context, "acquired" / "gained" is probably more idiomatic than "learned". But since in this case the writer is emphasizing "learning", as I argued above, it becomes idiomatic. Context matters!
If one is given a task, one might at various times in the course of performing the task make use of "previously learned knowledge" (knowledge which was deliberately acquired before starting the task), "previous knowledge" (knowledge which had been acquired, but not necessarily deliberately, during some vague period that that preceded the task), or "newly acquired/learned knowledge" (knowledge which had been gained between the start of the task and the action which exploited it).
Outside of certain specific contexts, the phrase "learned knowledge" without an adverb would lump together two concepts ("previously learned knowledge" and "newly learned knowledge") which would be sufficiently different from each other that it would be rare to want to refer to both of them while excluding other kinds of knowledge such as inferences gleaned through repeated observation.
In the phrase "previously learned knowledge", each of the words adds something to the meaning.
Surely it is simply the use of the past participle as adjective, as in "previously seen footage"; a "previously discussed topic"; "previously known information"; a "previously mentioned idea"; a "previously prepared talk"; a "previously determined course of action"; etc.
In each one the past-participle is used as adjective.
"Previously learned knowledge" seems no different to the normal use of the participle.
