Zoom climb
"Everybody wonders why the PF pulled back and zoomed to FL380 and entered a deep stall."
I never read it that way. My read is that they were unable to climb because the temperature did not fall. Never any indication that they "planned" a climb.
I read NOT that they climbed, but they ascended, or, they were pushed, or, caught a severe updraft, and then wound up at 38,000 in a nose up condition, at below 60kts and thus literally out of control because at that time the controls would be inneffective.
Which is consistant with being in a thunderstorm.