All of that may be true, but the fact is that OotS is a comic strip first and a D&D strip second. That means that if "this is funny" conflicts with "this works by the rules of D&D," the former is going to trump the latter every time.
The joke in this strip is that Belkar went up a level, then lost it right away when he started celebrating. If he gets the level back via Restoration or whatever, that ruins the joke. Which is why I don't think it'll happen.