lomapaseo
quite right Dave, problem would only arise at level off. aircraft levelling off and possibly distracted by making a turn, easy to miss that one throttle is stuck in the climb position. Even easier if the turn was to the reducing engine. This would also exacerbate the induced roll and yaw caused by the opposite engine on climb power.
Just to be clear, this would be the scenario.
assuming right throttle stuck in climb. Aircraft starts to level off, only the left throttle would retard. Pilot makes a turn to the left, he may not notice the right throttle is stuck. Aircraft makes the turn, pilot thinks all is ok as the engine would cause roll and yaw to the left. It would only be after a few seconds or so that things would start to get interesting, then the aircraft would quickly enter an upset. With the aircraft in IMC, pilot not fully up to speed because of the pandemic. The pilot would have to be pretty quick to make the corrections. Recovering from this type of 'powered’ upset, only relying on instruments, requires training. Unless the throttles were closed the aircraft would never recover. - Spin recovery technique.
The Captain was more used to flying the NG by all accounts. A/t system is much more reliable, probably not paying much attention to the throttles.