Ground Speed
One hypothesis is that the PF pulled stick back because of loss of ASI and fears of a dive and over speed - possibly reinforced by the stall warning later.
Whilst ASI was not available the ground speed could have been used to gain a qualitative idea of going faster or slower e.g. seeing GS decay 400kts to 1000kts as he pulled up and began to hear <<STALL>> then stick forward and see GS increase and warning disappear.
Clearly GS is a terrible quantitive measure of AS on its own e.g. a nose dive would give a low GS and high AS, however combining it with attitude, altitude, vsi etc it could have been of use.
We know he should have followed UAS procedure however the PF appears to have chosen to chase a response from a/c to his flight control inputs. These inputs became more extreme as he dug a deeper hole in seeking response. Glancing at ground speed could have helped him observe the input/response and get some feel in the void.