With the airbus' FBW [fly by wire] system and passive stick, the crew would have none of the force or buffet cues through the side-stick that might have told them this.

I am very far from an ATP and I know next to nothing about Airbus control systems. But, after about 9 years of hangar flying with some ATP's, it's my understanding that even fly-by-wire a/c have "shakers" in the control yokes. These are designed so that the pilot gets a physical cue that a stall is imminent. Maybe it malfunctioned or is not part of the Airbus design (if not, that, in and of itself, would be alarming).

This is very dangerous flying, I don't care who you are. In IMC, you're taught to "trust your instruments" because, well, that's the only thing that's going to tell you if you're upside down or not. It's a hell of a thing to be in hard IMC and not be able to rely on your instruments.