"Good alert defensive drivers will get twitchy straight away; "
I'd not even go that far. I'd say most drivers following at the same speed as the car in front would immediately be aware there must be a good reason for the leading car to be changing lanes on what in the video was an otherwise clear road, not a junction. The leading car, at best, was pulling out to pass a slower vehicle. A human driver would have known this and almost certainly have started reacting as soon as the indicator came on.