The problem is its not actually open. Watch his three mechanics, he keeps his arms really low and releases his shot really low. Its ridiculously accurate but anyone even close to his size can block it since he has such a low release. So if someone is within 5 feet of him and running with his hands up it isnt actually an open shot based on where his release is. It would be more like Patty Mills being rushed by KD and trying to shoot a 3 over him.
Matt has added his little dribble forward move as the person flies by trying to block the 3 to compensate for that and it has been successful. If he could master a crazy Manu style fake to get them to leave their feet and do a side step he would be killing it.