to the best of my knowledge, 447 never 'lost' a valid AoA signal?
The ADIRU determines the vane angle from the sensor 'resolver' values. Possibly it applies a calibration of vane angle vs body angle of attack for the flap/slat configuration. If the airspeed is less than 60 kt CAS, the ADIRU transmits the value NCD (No Computed Data) instead of the AoA value (as I understand it). The stall warning function resides in the FMEGC which uses the AoA values it receives from the ADIRU's.
The stall warning was lost after the captain arrived in the cockpit.