Human Factors
Can't help but wonder if this isn't a simple human factors issue. ASI has been reduced to a small tape indication on the display. I can't believe that the PF would have allowed the airspeed to continue to decay if he knew what it was. So, why didn't he know?
Airspeed needs a more prominent display.
FO in the jumpseat calling "sink rate" (should this rumour turn out to be true) just distracted the PF from the real problem, which was airspeed. He might have responded to the sink rate call with the pull-up seen on the video, which put the AOA so high that the subsequent (and too late) power application had no effect.
We'll get the answer pretty quickly, but this has to be autothrottle function misinterpretation plus situational awareness caused by HF. Possibly also a heuristic error due to previous A320 experience.