^It really depends on your definition of "done." One can only presume that in this particular case it means everything (level design, animation, dialogue, gameplay mechanics etc.) is "in place" and in a playable Beta state.
It might be worth keeping in mind that the sequel must have been in at least the planning stages before the first game was even out last year. For those who don't know there's a secret room the the warden's office that has a blueprint of the proposed 'Arkham City' project.
Given the quality of the game I think they would have been very confident that had a guaranteed hit on their hands and immediately made a start on the sequel. Just seeing how soon that original teaser came out should tell you how fast they were moving on it. So when you think about it it's not so surprising that they managed to develop it so fast.