it doesn't prove their idea is totally wrong, only that reliance upon it would be.

As that bit shows, I agree completely. But while evolution is correct, you can't use it to go around making broad factual inferences. While you should believe in evolution, you shouldn't go around making statements like, "There are no homosexuals," or "Every behaviour is adaptive in a fairly obvious way," just because your theory predicts it. This exercise properly demonstrates that while the theory is true in a general sense, broad inferences based on a simplistic model of it are not appropriate.