If you're comparing to other accidents in the same (geofenced) zipcode, then why is it not an accurate comparison?
The linked study even calls this out: the zip codes they controlled for are the residence zip code, which means accidents that happen on the freeway (where Waymo typically doesn't drive) or even in other zip codes are included in the human baseline.
Precisely.
In Phoenix, the waymo drive on the freeway around the city, and given the number of crashes I see in that city I have few doubt that Waymo cars are safer.