Is it possible that the remaining Pilot in the flight deck was incap, and the other crew on the aircraft simply forgot the appropriate code to regain access to the flight deck?
Highly unlikely I think for an experienced crew in a respected operator. Certainly in my airline, we regularly train for this and I sometimes ask my FO at the brief the procedure to gain access to the flight deck In these circumstances.
This would however alievate the theory of the pilot outside being ' deliberately ' locked out. Such a horrifying thought. . . . . .