Terry sinks in the one arm drill since he’s not wearing fins like the swimmer doing the “SS unco drill” wearing fins. So you’re putting a right and wrong context, one wearing fins and one not which itself is misleading at minimum. If you have to wear fins to perform a drill, only masks an imbalance issue created by the drill - and then what’s the point of the drill? So again, what is one becoming aware of using this drill other than creating the impulse to pull?
The single arm fly you posted too creates the impulse to pull with the opposite arm floating below the surface next to hip, may as well be single arm free with some fly undulation. The single arm fly I use with my swimmers, opposite arm remains in front to maintain balance as the high side arm swings forward from the pelvis accessing external forces of the weight and momentum as it swings forward and thus minimizing and being aware of primal impulses to pull to stabilize the vessel.
You noted you are an engineer. Curious, what type of engineering do you practice?