That wasn't a flash bang, that was a door breach. Shaped charge on the door blew that fucker wide open (see the door @ 00:03:28).
Remember too, they had been sitting on that room for an hour already. The shooting had ended a LONG time ago, and they were getting no response from inside. Shooting started at 2205 to 2215, and first officers were on scene at 2217. The door was breached at 2320, over an hour after the shots ended. They had that room under constant observation for so long, and not a single sound had come from inside. They had good reason to believe the shooter was dead or gone from the room. Even if the shooter was still alive/present, he would have had the opportunity to barricade and prepare for assault. He would've slaughtered an entry team at the doorway.
I'm actually against dynamic entries on non-assaulting suspects. Time is our best asset, and if we control that, then we control the scenario. Oakland PD learned this lesson the VERY hard way in 2009.