It’s all over CNN. They just told exactly how that door mechanism works, with picture. It has to be a deliberate act to lock someone out, so that the access number keyed into the keypad won’t work. There are three lock positions: “Unlock” “Norm” and “Lock”. The lock position is usually in “Norm” so if one crew member leaves, he or she is able to override the locked door through keypad access. The person(s) in the cockpit have the ability to disable that function by moving the lever to “LOCK”.
There is also supposed to be someone else from the flight crew in the cockpit if one pilot leaves, as there is always supposed to be two people in the cockpit.
Don’t forget about investigating the rest of the crew. Suppose the one who may have temporarily replaced the pilot who left had terrorist/suicidal intentions, took out the pilot who remained, locked the cabin door, and intentionally flew the aircraft into the deck? This would require them to have A320 piloting knowledge I suppose. We live in very strange days indeed.