Death of the Author is not about canonicity, nor is it a "fan-made concept". It's a concept that's an integral part of post-modernism, which is a movement far predating fandoms being a thing at all.

What is in the book (or whatever text we're talking about it) is undeniably canon. But every story has elements of ambiguity, and what the author thinks about those points of ambiguity is completely irrelevant, since everyone's interpretation is as valid as theirs at this point. When the work is out there, the author's perspective is but one of many. Even if he claims he has the right (like the creator of the .gif extension regarding its pronounciation), he has no right, really, to change people's perspective on it.

I think what you're referring to, Seph, is more likely Fanon Discontinuity