This kind of simplification happens in English. Neither the full form of the sentence nor a simplified version using a pronoun are necessary because the sentence is being used in relation to a specific and previously defined object. In this case, you're reading about the definition of the word "tiresome." That means the sentence can only be (or should only be) interpreted in relation to that word. This means you can simplify the sentence by dropping the object reference.
Full reference: Do not confuse the word tiresome with tiring...
simplified reference using a pronoun: Do not confuse it with tiring...
Simplified reference dropping the object: Do not confuse with tiring...
All three of those are correct.
Now that you know what's going on, listen/watch for it. You'll discover that object references are simplified regularly.
But I'll give you credit for this: the sentence with the dropped object reads fine ... but if you were to say that to someone, it would sound funny. I suspect that's because spoken English can't guarantee the object reference as well as written English. Were someone to speak that sentence, I would expect them to use either the full reference or, more frequently, the simplified reference using a pronoun.