Ancient books are found from the Sage's time and everyone assumes them to be historical texts.

Obviously Kurama is Kurama.

EDIT: Let me explain. They find YYH, they think it's a historical document, it's not. They, or some one, thinks that the Kyubii is Kurama. He's not. And is possibly insulted, (or wants to know what manga volume it is, because his collection is missing bits...)