Ok
"Waymo's fleet has not yet driven enough miles to support that they are safer than for hire vehicle drivers."
If there's not enough data to show they're "safer" than there also isn't enough data to show they are "less safe".
To scale it in human terms, say that you're a human driver and you're 2x better than the average human driver. If you wanted to prove that you're better than the average driver, you'd need to drive multiple hundreds of millions of miles to statistically demonstrate so.
Given that the rate is significantly lower for taxi drivers in aggregate, to prove that they're safer than taxi drivers if Waymo is 2x safer, they'd still need to drive multiple billions of miles to statistically prove that they're safer.
That seems like the same statement, unless we apply stochastic class level subtlety.
This is not that simple. Emergency service vehicles are at a very high risk of collision when they pull out of the station and crossings close to the station, or when leaving a situation under time pressure. Busses are at an increased risk of collision when pulling away from a bus stop. Construction vehicles are at a high risk of collision when they pull away from a construction site at a busy road.
Hence why you need more data, after which average accidents per mile are a much stronger indicator.