If you do want to extrapolate from their data, it would be worth looking at the total number of accidents--not just fatalities. Insurance companies say people file an accident claim every 18 years on average. If the average miles someone drives each year is 12,000 miles, this means they get in an accident every 216,000 miles on average. If Uber drove 3 million miles, we should expect them to have been involved in about 14 accidents over those 3 million miles if the cars are on par with humans.

(I'm guestimating on some of those numbers, but the should be somewhere in the ballpark.)