They should have provided some mechanism for the real owner to approve updates if the updates aren't all trusted by default.
You could do a PIN/password, but if it is never used during operation, nobody will know it. Ask anyone who’s had a head unit that needed a PIN after losing power.
Agree that a PIN/Password would have usability problems with a car. Since no car manufacturer intentionally permits you to install software you want, there's no standard mechanism. But if this was standard I think an owner-set PIN would be very reasonable.
Otherwise they would have had something like an unlockable bootloader where you need a special key to unlock it, or something difficult to access switch or something like that.