>Can’t think of any reason that might be a problematic hair-trigger response for a driver.

It wasn't a knee jerk response. It was a response he had on tap based upon level of situational awareness. The car pulled out poorly but he had been keeping enough of an eye on it that it that he was ready to intervene and just a "haha imma cut you off and then flor it because EV rocketship" asshole move rather than a near accident. No different than keeping tabs on something in your mirrors so you're ready if it develops in a particular way that causes you to want to take action. Or being ready to lane change as you approach a light in case the person in front of you decides at the last minute they want to turn there.

If "brake pedal is magic button that makes all situations better" drivers drove on his level everyone's commute would be better.