Under normal conditions they can by using a special code, but this can be overidden mechanically. It pretty much rules out the rapid decompression theory because the door automatically becomes unlocked in such an event. Knowing what was going on and and how this was going to end, then may be he should have tried kicking a window out to force a decompression.
The last 8 minutes of the flight would have been terrible for those onboard, especially watching some trying to smash the door down before been slammed into a rock face at approx 400mph!