Why the backstick?
The consensus seems to be an incredulity as to why on Earth the PF held backstick? None of us would ever do that.
Speculation: PF felt the stall warning was triggered by a 10000'/min downdraft -- and just before, he felt he had hit a 7000'/min updraft that he had countered with forward stick. Perhaps PF hung his hat on his windshear/terrain avoidance training.
When in a rapid descent, a low airspeed indicates stall which requires stick forward while increasing airspeed indicates spiral dive which requires wings level and stick back (leaving out the finer details of spiral dive recoveries for sake of brevity).
But here the crew does not have an airspeed worth looking at as it is flopping back and forth.
Recovering from an upset at night in IMC is demanding enough. When you don't have a believable airspeed to distinguish between stall and spiral dive, you have a 50/50 chance of getting it wrong.
In normal weather in a spamcan or glider, you can hear increasing airspeed. Not so easy in a CB.