Bless her. I've been shipping Dumbledore/Grindlewald since I read DH. And now I'm wondering how many of us will still be so eager to say "Author interviews don't count," without adding, "...er, unless I want them to!" which has pretty much been my stance all along.
And this is one I wouldn't have expected to see revealed in the text, either, because it wouldn't have furthered the plot, it'd've stopped it cold. No place to put it. And that's saying something in a novel as scattered-structured as DH, whoof. Hard to admit that sort of revelation--a theme that ordinarily would have made me squee--would probably have had me yelling, "Shut up already about Dumbledore's effing history, woman! You've bored me to death with it already!"