Being rear ended (16 out of 23 serious accidents according to the article) is a pretty clear case of the car not doing anything wrong at all. It's the one case where collision avoidance is going to be useless because the car is waiting for e.g. a red light and is supposed to be stopped and has to blindly trust cars behind us will do the same thing.
Your assumption is that it was stopped at a red light. What if if slams the brakes on in the middle of the road due to a mylar balloon, etc? Does it sense a vehicle approaching quickly and sound the horn to hopefully alert the driver to stop, and pull forward from the stop line into the crosswalk if it's clear to provide extra braking distance?
Hypothetically possible but the article suggests these were situations where Waymo was not at fault. And I actually know people that have been rear ended at traffic lights or junctions. Twice. In the same month. Both times after they came to a full stop in a spot where they were definitely required to stop.
There's nothing you can do when that happens. Some idiot coming in way too fast not paying attention for whatever reason. Stuff like this is quite common. I don't see how you could mitigate that easily.
I've mitigated it in the past. You sound your horn, which can cause them to look up from their phone, etc. You can also pull forward through the crosswalk if it's clear. If anything, Waymo sensors should be better able to identify vehicle speed delta to sound the horn when a vehicle is approaching it too quickly.
