It just seems that for A/S to truly drop so low so quickly, a lot of other things would need to be happening at the same time. The 787 avionics architecture is so integrated and redundant, I don't see why such drastic changes in A/S can't be checked against inertial attitude and acceleration. Likewise, I'd think a pilot would know that such a change in airspeed, if valid, would be accompanied by some very noticeable vestibular cues. Lack of such cues has be a cue that the airspeed has indeed not changed so drastically.