What they of course should do is count any manual intervention as a possible autopilot accident.
When I say possible, what I mean is they should go back, run the sensor data through the system, and see what autopilot would have wanted to do in the time that the human took over.
There are a couple reasons why your criteria would get almost entirely false positives.
First: Most Tesla owners disengage autopilot by tapping the brakes or turning the wheel. This is typically more well-known and more convenient than the official way to disengage autopilot (push right stalk up, which if you do twice can shift to neutral).
Second: Tesla autopilot drives like someone taking a driving test. It accelerates slowly, signals well before changing lanes, makes sure there is plenty of room in front, etc. In my experience, the vast majority of interventions are to drive more aggressively, not to avoid a collision. I think maybe twice I've disengaged to prevent a collision. In those cases it was to avoid debris in the road, not another vehicle. (The debris was unlikely to damage the car, but better safe than sorry.)
Those interventions are to get somewhere faster, not to avoid a collision. If anything, such interventions tend to increase the risk of collision, not decrease it. Training autopilot to behave more like humans in those situations would make it less safe, not more.
If the intervention was not to avoid a collision, they review the footage and find that autopilot would have done something safe, and therefore it is not deemed an autopilot accident.
That's a bit speculative, since your actions will affect the actions of others, but I agree if it were done correctly would give the best picture of autopilot safety.
When I say possible, what I mean is they should go back, run the sensor data through the system, and see what autopilot would have wanted to do in the time that the human took over.