@ RobBamber
It seems to me that, given no visual cues nor instrument readings, a pilot flying by the seat of his pants may interprete a pitch up attitude coupled with such a horizontal deceleration as level flight.
Didn't the PF acknowlege reaching level 100?
Quote from
http://www.bea.aero/fr/enquetes/vol....ai2011.en.pdf:
At 2 h 13 min 32, the PF said "we’re going to arrive at level one hundred".
That's roughly 25000' from the level 2 min before (350):
"At around 2 h 11 min 40, the Captain re-entered the cockpit. During the following seconds, all of the recorded speeds became invalid and the stall warning stopped.(...) The altitude was then about 35,000 ft, the angle of attack exceeded 40 degrees and the vertical speed was about -10,000 ft/min. The airplane’s pitch attitude did not exceed 15 degrees and the engines’ N1’s were close to 100%. The airplane was subject to roll oscillations that sometimes reached 40 degrees."