I would have guessed that based on the distance of the flight, this plane should have had three crew. Assuming that the captain was on a rest break rather than a bathroom break, that would mean that two FOs in cockpit were sleeping or otherwise didn't notice the descent.
Only a few years ago, might still be the same, under Italian FTL's "Augmented Crew" the autopilot was/is considered to be the third pilot so two (human) pilots might be working 16 hour or longer FDP's.