I should imagine there is a key code access to the flight deck. If the FAS type it in, the door alarm goes off and the door will temporarily unlock for a few seconds then re lock so long as the door is in normal mode. If it is in lock mode then the door will not open period.
All airlines have different equipment. It would need a source that KNOWS the MH 777, to rule out the loo theory (as it happened on LAM E190 a few months ago = deliberate CFIT).