Humans drive in all weather conditions on all types of roads and also many types of personal vehicles of varying ages and conditions.
Waymo is limited to few specific locations with decent roads and does not drive in poor weather and is limited to a relatively large and safer expensive SUV that is maintained professionally in a fleet.
Studies like this rarely account for such factors , they are compare optimal conditions for self driving to average conditions for humans.
Even if waymo was better when accounting for these factors , if it was much worse in the conditions humans typically are expected to drive [1] they self driving is still less safe than humans on average .
A better comparison could be with professional taxi drivers for the same city (not Uber or Lyft).
I wouldn’t be surprised if Waymo is either on par or poorer than this group .
[1] no study will ever show this as they wouldn’t be able to trial it under those conditions if it is not safe enough