riddle me this; how do you expect Bonin to have a feel for what the "normal" roll gains are and therefore be thrown by the difference in Alt2B when it is on record that he had no training and likely little or no experience in manual handling at high altitude?
Dozy, very simple. He was expecting a Normal Law type roll response. What he received was very different and much more vigorous. The scanty information I've seen on his training indicated that he had never flown in Alternate law at altitude. That does not exclude flying in Normal law at altitude. I would also expect Normal law at altitude to fly about the same as Normal law at lower altitudes due to the intervening computer magic.
Bonin began flying when the autopilot dropped, before he knew the nature of the causal malfunctions and their implications.