Regular customers have to wait weeks to get their funds out, but the hacker managed do withdraw this in an instant. So of course, MtGox knows them. And so eventually somewhere in this MtGox account thread they stated that they do, but don't share this information publicly.
I don't follow your logic. The hacker logged into MtGox as Bitcoinica. MtGox allowed the instant transfer because they know who Bitcoinica is. This doesn't mean they know who the hacker is. I have not seen anywhere that MtGox said that they can identify the hacker.