Instead of declaring the answer simply wrong, it would be better to say that the answer is missing details. Indeed, nowhere in the answer it was "assumed" that $f(x)=ax+b$. It was "proposed" that $f(x)=ax+b$. One can legitimately ask, how did the answerer get that (such a method is elaborated
here). The point is the answer needs to be filled up with details but that doesn't make it "simply wrong", does it?