Are you sure? Again this is just how I'd do things, but after I had a bunch of disengagement snapshots, the next thing I'd do is start taking snapshots of similar situations. We live in an age of easy and super-accurate image classification, it wouldn't be too hard to start a snapshot every time the front camera view contains a roundabout.
What I'm trying to say is that the number of cars on the road and miles driven aren't what matter to their autopilot learning, when compared to other self-driving companies. Most people use their car to go places comfortably, not to look out for learning experiences for Tesla.
With how long commutes are in the US, a very very small fraction of miles driven by people on autopilot will be other than "the car was already on the highway, autopilot changed lanes a few times, and after a while was manually disengaged to take an exit".
That small fraction is what could be compared with the miles and number of cars driven by the other companies, because in the latter case it's hired drivers actively looking for conditions of interest to the engineers.
