Door swings inward and any pressure over 1 atm inside the fuselage would render impossible an in-flight opening of the door --they would have to be near sea-level with no cabin pressure at all.
Not impossible, but still hard.
People are saying the passengers would resist, but would they need to be WITTING to the take-over? Not necessarily, if the pilots OR crew were in on the take-over.
The flight was in the middle of the night, so my guess is many would be asleep over the course of the 4-hour flight and would have very few visible points of reference over the ocean.
Unless there had been a FIGHT to enter the cockpit my guess would be that they'd go to sleep totally confident of waking up in China.
The turn around would have been unmistakable. It happened an hour or so into the flight.
One or both of the pilots would have been involved in this.
If a hijacking, these guys were good. They made a 777 disappear.
Hope I am wrong.