This is something that's come up recently with the quote about Fallout and capitalism. And it reinforces the idea of "death of the author", the idea that any work should be evaluated on the text alone, and any interpretation is valid so long as it can be backed up with the text. Furthermore, once a work is completed, the author's interpretation isn't given special consideration or weight.
To believe otherwise implies that a work by a person who's died is necessarily incomplete and cannot be properly interpreted or understood.
So when JK Rowling tweets that Dumbledore is gay, well that's just fan fiction.