Their argument is that comparisons against the driving record of cab drivers better represents what they should expect from waymo vs comparing them against every kind of driver everywhere. Considering the driving skills of some cabbies I've ridden with I'm surprised that cab drivers come out ahead of the average driver, but I'd agree that a lot of that is probably thanks to fewer highway miles.

Waymo compares its safety record with that of the average human driver. The study says New York City's for-hire vehicles are involved in serious or fatal crashes at a rate that is 14 times lower than the average vehicle, making them a more appropriate benchmark for robotaxis. ... The researchers compared Waymo's nationwide crash data with New York City's for-hire fleet and found the robotaxis had a serious injury or fatal crash rate about 1.5 times higher per mile driven.