I don't think this is the correct comparison. A car used correctly is safe. We have huge numbers of road accidents because most people are unable to reliably use a car correctly. The value of an 'autopilot' functionality is that it should be much better at using the car correctly in the real world than a human.
What matters is not how many accidents result when using autopilot 'correctly', but how many accidents result from using autopilot in the real world.
Also, because autopilot is primarily used on particular road profiles, it's not fair to compare accidents per autopilot mile directly with accidents per human driver mile. You need to adjust for the fact that autopilot is not used during more complex driving anyway.
I'd be very interested to know what the statistics are for those, since the recent press has given me a (potentially incorrect) impression that autopilot has lead to a relatively large number of serious accidents compared to the number of cars deployed.