That’s probably how they do it, which is again very clever stuff, chapeau. But they do it like that b/c they can’t really predict the world around them fast enough. It might be possible in the future with AI World Models though
Even if you detect “fast enough”, there are physical limits for braking and coming to a stop.
Do you stop at every double parked vehicle when you’re driving? A Waymo would never move if it always predicted pedestrians would jump in front of it out of nowhere or the car next it would swerve at 65 mph. It’s physically impossible to stop in time for many accidents, unless you’re already stopped.