So the important point to discuss is why they did not know
Read some posts below to get the typical pilot misconception :
All aircraft have a stall speed, Cessnas about 45 knots, Airliners about 110 knots.
Not totally wrong (al least under certain circomstances), but still the wrong concept. All aircraft have an
AoA where they stall. Under the given conditions (weight, air desity...) this corresponds to a stall
speed.
Many pilots understand the stall to be a
speed problem, it is not !
So if you think stall is a speed issue, and you know your speed indication is invalid, it is very, very easy (and maybe it is the natural behavior of humans to seek for the most easy answer in a panic situation) to assume that the stall warning in that case is invalid as well. Especially if the noise of the airflow at high AoA (supersonic around the cockpit) makes you believe that there is enough speed and your indication is still invalid.
keep the side stick all the way back
Read the report carefully again please. Unly when already deep in stall the pilot kept the stick full back. To get into that (most probably not recoverable) situation the pilot did
not apply "stick all the way back" unless for some very short moments.