What I think of the post as saying, rather than "purpose has only the meaning (to english speakers) of a ternary relation," is that "when one normally asks about something's purpose, one implicitly uses its structure as a ternary relation, and since you haven't established a ternary relation here you aren't going to get a satisfying answer that way."

I think I agree with you on at least one point, though, which is that "words" are really not the problem object; the sentence "what is the meaning of life?" is grammatically correct and not logically invalid and is somewhat a different use of the word purpose. The core object in these constructions I think is cognitive algorithms; in particular the "hear the word purpose, search for Z" algorithm breaks down when purpose changes meaning to no longer involve the same sorts of X,Y,Z.