Authorial intent doesn't mean much. It can be used to support an argument discussing the author's work, but it cannot be used to invalidate one. The reason for this is that everything one needs to analyze and discuss a work is in the work itself.
Imagine that the director of Birth of a Nation says that his movie is not at all promoting white power, and that he had no intention of pushing a white power agenda. Does that suddenly force the intellectual community to conclude: "Welp, pack it up, boys. We were wrong, and there is nothing to discuss here!" No, it does not.
Your questioning has been answered a long time ago by a Frenchman called Roland Barthes in his essay
La mort de l'auteur (
The Death of the Author). I invite you to read
its wikipedia page, at least. The essay itself also, if you feel like it.