They are ordered by "when did they last have a post" (any post, not my post). This is correct.
UNLESS my last post was more than... some time ago?
Or is it UNLESS the thread as a whole had a quiet period of at least.... some amount of time?
This isn't wrong necessarily. I can see why you might think people wouldn't want to be bothered with reanimated threads, but my personal preference would be to have them reappear.