up to the exit from the flight envelope, the airplane’s longitudinal movements were consistent with the position of the flight control surfaces,
this sentence does not mean that they don't know the correctness of flight control surfaces movements, but: You simply cannot predict the flight path in a stall anymore, the aircraft is not behaving "aerodynamically correct" or "like designed". It rather moves in a random pattern, depending on chaotic aerodynamic laws, not predictable and not rationally explainable (unless many more factors would be taken into play, in a much bigger "resolution" of values and mathematical formulae).