Also if he will delete all his posts I am sure if someone wants them, he should only send a PM to an admin (and most probable those posts will be re-added).
I have seen this behaviour previously. It's probably a SMF bug.
His threads probably shouldn't be in the trashcan. He edited most of the OPs in threads he created to make it misleading as to what was being discussed (and he moved them to incorrect sections).
I don't see the point. He is taking a lot of pains for nothing.
He might have used a bot?
It is not a bug, because the activity (so also the ranks) is updated every hour.
I have done this before, when I was Jr member, I deleted few post, and after 30 - 60 minutes my rank back to newbie.
So it is only a question of 'time' and he will lose his Hero rank, I am waiting to see the change.