It feels like it would be useful to also know how many fatalities would be expected from Lyft in the number of miles Waymo has traveled based on their calculated rate (which should be fairly straightforward to calculate with only the rate you gave and the number of miles Waymo has traveled, although I'm not sure if it's known) and the probability that Lyft would also see zero fatalities with that number of miles traveled (which I think would require more detailed knowledge like variance, although I admit I haven't spent enough time to convince myself with complete confidence that this is correct).

I imagine it also goes without saying that not every mile of road is equally risky, and I have to imagine that Waymo's miles traveled probably on far less risky roads on average given the way they've been rolled out (which isn't a bad thing, but it does make extrapolations from the data about relative safety a bit more dubious).

I think you’re spot on.

Unfortunately stats can be spun in whatever way you need them to in order to support or hinder an effort.

To use your example, risk per mile is a sliding scale, not necessarily a Boolean value. So then someone could conceivably draw the line on that scale wherever most benefited the company they were trying to support.