So, you're right, there's no clear distinction, but I would further argue that it doesn't matter. Even if only 1/1000 disengagements are fatal, my conclusion remains the same. I think we're splitting hairs at this point, though.
Even if not fatal, I highly doubt a significant fraction of such events (as defined above) would allow me take a nap upon departure and wake up at my destination, so it would still be unacceptable to me. I guess we have to agree on what an acceptable end-game is for fully autonomous vehicles. If you think "waking up on the shoulder exchanging insurance" is acceptable, then that would indeed change the numbers (but by how much? Two, maybe four orders of magnitude?).
Humans get into fender-benders all the time, but surely we'd strive to eradicate this inefficiency in the automated driver. I think this is still an active area of debate; some assembly-line work can be made more efficient with machines, but we've seen humans out-perform machines in other types of work. I think driving tends to utilize more reactive, intuitive "System 1" thinking[0], so I imagine that humans will be vastly better than machines at driving for a very long time.
And no, it's not acceptable by any stretch. A disengage is extremely dangerous to drivers who are lulled into false sense of security through the marketing of.. self-driving cars.
This doesn't seem reasonable - Waymo's report doesn't dive in depth enough about each disengagement to warrant this sort of extreme reliability.
If "2 disengagements" per year, were at most fenderbenders - something I'd wager humans do way more than twice per year - that would be a very different story than if those 2 disengagements were life threatening. Sure you'd wake up from your nap, but you wouldn't be dead, and at most you'd have to exchange insurance information.