The story here is how the information came out – at a book reading/signing – and, if a fan there hadn’t asked the question, people probably still wouldn’t know for sure. In fact, since the release of the seventh and final book in the series, Harry Potter and the Deathly Hallows, Rowling has, here and there in interviews, revealed pieces of the story that either weren’t included or were just alluded to in the books.
It would be interesting to find out if there was just too much information that needed to be addressed and there wasn’t enough room for everything in the series, or if the author purposely left out some tidbits to keep the story going after the final book’s release. Either way, it should keep the Harry Potter buzz going well into the release of the final two films and begs the question of what exactly Rowling will reveal next.
It also will keep interest alive if Rowling chooses to write, as she’s said she might eventually, a Harry Potter encyclopedia, which should offer a chance to disclose any other left-out details and clear up any lingering questions.