Uhm, that's vertical speed. The problem is the plane slowed down to the point it couldn't maintain proper airflow over the wings. Thus the stall warning.... and well, the stall. Unexplained is why the pilots kept the nose of the plane up. I would imagine at some point that would cause a spin. The recovery procedure for a stall is full power and nose down until your airspeed comes up again.... your forward air speed.
My only thoughts is that the pilots didn't believe the stick shaker since they were pretty sure the speed sensor had failed. So when they started to feel gravity again, they had assumed they had recovered from the stall. Amazing they didn't just turn it into a flat spin... or maybe it was...
Bottom line, without an indicated air speed, its is extremely difficult to figure out how close you are to death. Note that the air speed pitot tubes sometimes double as air pressure indicators..... which is used to get altitude as well. I thought they would have backups... but maybe not.
Wait for the final report.