This was after he put up a replacement level season, not after a league average one where he can play a passable shortstop.
I'm very puzzled by this deal, it seems that even if they completely love Thornton that they should have been able to get more added to that deal. On the surface he's a low walks old starter who potentially discovered some strikeout ability this season, but not enough to blow you away given the age/level, and has a very weird delivery which I honestly don't really care about unless that's a detriment to his health for whatever reason.