They can restore them if they want. A guy here was a kind of big user in some tech support sub. He didn't just delete...he used one of those account scrubbers to edit over all his comments a few times before nuking his account. Went back to look a few weeks later and all his answers were back.
They don't do this with everyone because most people aren't important to making a sub look useful and attractive but...yeah. They have everything we all ever said stored.