This doesn't have to do with communicating with the car.
I believe internet is needed because tesla acts as a CA for your phone. That allows you to revoke a device, for instance if it were stolen. My guess is that the phone has authorizations that it renews fairly frequently and in this case apps could renew auths so once the current set expired you couldn't use the phone to unlock, etc. I can't immediately think of a better solution to the problem that allows using a phone for some functions and being able to remotely revoke a device.
If letsencrypt went down we'd similarly see a large portion of sites be inaccessible through browsers after 90 days even though you could directly connect to those servers
I believe internet is needed because tesla acts as a CA for your phone. That allows you to revoke a device, for instance if it were stolen. My guess is that the phone has authorizations that it renews fairly frequently and in this case apps could renew auths so once the current set expired you couldn't use the phone to unlock, etc. I can't immediately think of a better solution to the problem that allows using a phone for some functions and being able to remotely revoke a device.
If letsencrypt went down we'd similarly see a large portion of sites be inaccessible through browsers after 90 days even though you could directly connect to those servers