An opt-in switch is merely convenience for the incredibly thin % that bothers with this kind of thing. And that % will actually be informed enough to not opt in.
Come to think of it, that % will likely be informed enough not to buy this kind of device in the first place.
Tesla has a pretty vested interest in shit keeping working considering it's a pretty luxurious and high-profile product. The cut-price manufacturer of your $20 lightbulb or $300 fridge? Not so much.
It's not a matter of luxury, it's a matter of having people work on it who care.
Disclosure: I work with them. Much <3 :)
Source: Chromebooks are awesome, and even with excesses like this, they're still the cheapest to operate by far.
A comment in https://bugs.chromium.org/p/chromium/issues/detail?id=323010 claims that the BSSID is used for a "suspected" state, but that may not be enough to actually stop it from downloading updates, but I'm far from an expert in that domain.
In short, identifying tethering states with iOS seems to be hard.
How does Tesla implement updates?