The guarantee here is that the human isn't obligated to intervene for a moment.
If you call that guarantee impossible, then what about level 4 cars? They guarantee that the human isn't obligated to intervene ever. Are level 4 cars impossible?
Is this a wording issue? What would you say level 4 cars promise/provide? Level 3 cars need to promise/provide the same thing for a limited time. And that time has to be long enough to do a proper transfer of attention.
Ah, understood. So the guarantee is that the driver is not legally responsible for anything that happens in those 10s. I always took that as a guarantee of safety rather than from legal consequences.
The guarantee is that you will be very safe and you can go ahead and look away from the road and pay attention to other things. But at most this is as good as a level 4 or 5 car, not an impossibly perfect car.
When dealing with unpredictable real life events there are no guarantees, unless we're considering the many carveouts to that definition from a legal perspective. A blind car (fluke weather, blown fuse, SW glitch, trolley problem) can no longer guarantee anything. Giving the driver 10s, or assuming the worst and braking hard could equally cause a crash.
> your transmission suddenly exploding is an exception
As long as the brakes or steering work a driver could still avoid a crash. The driver having a stroke is closer to a blind car.