Dozy "Well, this is the thing - we've had two explanations doing the rounds - one, that the stall warning is inhibited by a software setting and the other is that once the airspeed has fallen that low that the AoA vane no longer functions. No doubt we'll find out if one, the other, both or neither is true in the coming weeks and months."
Don't forget that with a very high AoA the airspeed indication is VERY spurious. You get some ram air into the drain and you get reduced air into the business orifice of the pitot. The AoA vane was probably working. And when the nose was 10 degrees down would have clarified the stall warning by showing the AoA was still way too high to fly.
(I suspect it would be wise to get that green line I've started harping about into your graph for thinking purposes. It may explain some of the cockpit psychology.)