It's allowed for topic-starters to delete all replies to their self-mod topics. Restoring the posts would only be considered in special cases such as if their account had been hacked.
Also, if a topic has no more replies, moderators will typically delete the topic on request, since then there isn't the issue of deleting other people's replies. That's why the empty topics were not restored.
It's just that he trashed a bunch of threads out of spite or mental breakdown or whatever that was, not because of anything related to the threads themselves. That's why I think it's abuse of a forum feature/privilege.
Perhaps there should be a time limit on self mod. I can't think of a good reason to delete posts months later.