We can distinguish between interpretive statements about the meaning of the work (which is what the Bradbury thing was all about), and questions about characters and story.

Imagine that Rowling's response to the fan's question were to instead say, "You'll find out in my next short story, due to be published in F&SF next month." And the short story was about D/G. OK, so let's say it was a short-short story. Like a paragraph. And then imagine Rowling read it from the podium. Why should her phrasing of it as an answer to a question rather than a short story matter?