Sensor validation;
Does the report say that airspeed fell below stall speed? IIRC it only stated that airspeed fell below VLS (lowest selectable speed).
The report explicitly states that overspeed protection was not invoked.
But does that explain the pitch response before the pilot inputs knocked it out of the protection mode? To control to alpha prot with falling airspeed why would it pitch up 10 to 16 degrees? Perhaps that's just what happens when it tries to AoA with very noisy AoA sensor inputs due to the turbulence? As the speed falls does the target alpha prot increase?
Capture of alpha-prot means that the FCS maintains an AoA greater than required for level flight, hence loadfactor>1, hence vertical acceleration, increasing v/s and FPA. Since pitch = FPA + AoA, increasing FPA at constant AoA=alphaprot means increasing pitch. Agree that turbulence 'noise' gets into the act too.
EDIT:: I suppose alpha-prot is tied to alpha-max which decreases with increasing Mach. So yes, I would expect that alpha-prot increases with decreasing airspeed, but consider that is a second-order effect. The protection will still keep the airplane a comfortable AoA margin away from the stall, because stall AoA increases also.