Every collision is avoidable if you drive the car at 5mph at all times. But that wouldn't be reasonable. If the Waymo description of the incident is accurate, there would be no way to really avoid this accident without significantly nerfing the normal and expected driving behavior of this vehicle. e.g., you would need to not allow a Waymo vehicle to enter the intersection when there is any other vehicle in it as well, which (in my opinion) would not be reasonable at all.

If it drives better than a human (in this case it sounds like it probably did), that's good enough for me.