Disengagements is a useful number, and the disengagement numbers say so much more about the real state of self-driving cars: That they fail as often as a non-synthetic oil change. And if humans had driving system failures that often, we would not be considered good at driving.
The longest number a company should be marketing is how many miles they've driven between disengagements. Like the "days since an accident" sign at a workplace: Fail, and you go back to zero. The numbers will be smaller, but they'll be a lot more meaningful.
Risk also skews results: Pilots taking off and landing on beaches in Alaska aren't comparable to pilots taking off from airports on the basis of incidents per 1000 hours.
i'm not sure what benefit there would be to gaming the disengagement stats. If anything, i think the incentive would be the other way around - inflate your number of disengagements so your competition thinks you're further behind than you actually are.
The point of testing is to bring a product to market, and if your product sucks people are going to figure that out relatively quickly. hiding your disengagements from the california DMV isn't going to do anything for you except build hype your product can't live up to.