I am sure this will be covered exhaustively but it does seem like a pretty "action movie" rescue on the part of the offduty pilots, the plane was in free fall for ~20 seconds and by the time they gained control to pull up, the force to avoid crashing and redirect all that momentum was so intense it broke a chunk off the rudder.

They claim the stabbed pilot came and opened the door for them while the other one was "trying to destroy the instruments in order to disable the plane", like he was just randomly smashing dials or something. Then the passenger that got into the cockpit dragged him out in a chokehold and the other passengers apparently tied him up "with his own zip ties and with the multimedia headphones", which is terribly convenient. Then the passenger flew the plane for a bit because he watches Air Crash Investigators, until another pilot appeared and landed in Tabuk.

the stabbed pilot managed to get it open. flight safety boards are now finding improvements to the system of accessing the cockpit. after 9/11 they made doors unrammable, after the german wings murder-suicide they made cockpits have two people at all times, and now this

Crew can enter an override code to unlock the cockpit, if the override is not canceled.

I think in this case, they were able to gain access because the cockpit door was opened momentarily.

that’s an international rule, not just an US rule. the cockpit door was opened because the stabbed pilot managed to open it

People willing to nosedive a plane into the ground don't tend to be in a great mental state.