I hadn't heard your 'authenticity' request before this article (that you want it only as an adjective, not as a noun). If I had heard it before, I would've contested it immediately. After all, the king of 'authenticity,' Walter Benjamin, didn't feel that it needed to operate upon a noun. He said that an object could have an aura or not have an aura. And that aura was totally a function of the object as a unique piece of art. Not as an object when contrasted to similar objects. Something can be inauthentic without relying on trope (genre or otherwise).

Probably, without hearing your answer first, I'm gonna guess you'd have no problem with someone using the word like that. Your problem is probably that when someone says Ashley Simpson isn't authentic, what they mean is she's not an authentic singer/songwriter or something like that. And you want them to make it explicit so that you can parse it. But it can also be used in the Benjamin sense.