One way would be for the perp to use every third or fourth magazine to spray down the door to discourage a breeching action.  Since no one was standing outside the door taking time measurements and recording notes,  its possible the shooter didn't fire 200 rounds thru the door at the beginning, and then turn his back on his most vulnerable direction for the next nine minutes.  The police likely know.  Having interviewed the guests.  But I don't think this had been covered in any of the briefings I've heard.