You have an intersection of ideas here: what adjectives are and how they function, and specifically the role they play in definitions (SEP), real or otherwise.
First, note that some adjectives, according to Barbara Partee, are considered subsective modifiers and some are not. From WP:
a subsective modifier is an expression which modifies another by delivering a subset of its denotation. For instance, the English adjective "skilled" is subsective since being a skilled surgeon entails being a surgeon. By contrast, the English adjective "alleged" is non-subsective since an "alleged spy" need not be an actual spy.
So, in the case of "false dichotomy", the question then really can be restated as whether false in this role is subsective or non-subsective which has obvious implications in a natural language ontology (SEP). Regarding "false dichotomy", it most certainly is a subsective modifier because a false dichotomy is not fictional or imaginary, but rather indicates leads to specious outcome in reason as it has the type "fallacy". From WP:
A false dilemma, also referred to as false dichotomy or false binary, is an informal fallacy based on a premise that erroneously limits what options are available.
Thus, the nature of a false dichotomy is that it also has type "proposition". For instance, "America, love it or leave it!" is a false dichotomy because it is an exhortation which presupposes two possible choices: loving or leaving, when one obviously can both dissent and remain.
So, a "false dichotomy" is merely a "proposition that contains a claim that there are necessarily two possibilities" when there are obviously two or more possibilities possible. What's important to get to our conclusion is, a "false proposition" still has type "proposition", and therefore allows us to conclude that no category mistake is being committed by the use of this modifier. The same applies to belief. Beliefs can be true or false beliefs, and in both case, the modifier is subsective and is therefore a subclass. Members of a subclass are still members of the superclass to which the subclass belongs.