Q is saying that he didn't think of that as a thing to do with a Dementor and should have. One possible reason for his "very odd smile" is that he did already think of it as a thing to do with a horcrux. I think it's reasonable to call this a hint that maybe there is a Voldemort horcrux buried deep underground. (Certainly far from a proof that there is, though.)
New chapter, and the end is now in sight!
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 103.
There is a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: