The sub-basement is his rabbit hole. He grabs his stuff, goes down there, and is supposed to escape through a way to the NN but he has never opened that particular way before, so has no idea what to expect. This does not eliminate the possibility that he has opened other ways out of the apartment. It is well established that the same room can have multiple connections to the NN.
I disagree that this is well established. The situation in the Deeps is the only example I can think of and I think that Cowl's portal came from somewhere beyond the NN.