I can assure you the people, who should know the code, don't necessarily know the code. Many airlines, routinely leave the cockpit single crew, when another pilot take a break or a comfort stop.
I can't go with that. I would be amazed if the cabin crew and pilots of a national carrier do not know the access code to the flight deck. My carrier changed the code once post 9/11.
Whether you subscribe to one of the pilots committing this act or not, the fact remains that the transponder and ACARS were switched off. Whether it's hijack or pilot suicide, those items were disabled for a reason as was the absence of a radio call to Vietnam on airspace transition.