I assume you are referring to this video: https://www.youtube.com/watch?v=sEpHyMtDcPY

> building a distance buffer", that is the proxy the insurance companies want to use for risk assessment.

The cam car did not need to have a hard braking event (HBE) to start building distance.

Even if they did, the insurance companies are looking for a pattern of HBEs to assess risk. I agree that there is a theoretical high-risk driver that never has a HBE because they always try to maneuver instead of breaking. There are other heuristics for this (high lateral acceleration, high jerk). And the ultimate heuristic: failing to avoid accidents, thus having a claim history.