Yeah, I'm a little suspicious about that claim.

Bluetooth tethering is a thing, actually predates wifi tethering. Though it's not enabled unless you enable Personal Hotspot in your phone settings (and Android requires it to be enabled separately).

CarPlay complicates things, as it only uses bluetooth to pair, then it switches to using a wifi network (as bluetooth doesn't have anywhere near enough bandwidth). Maybe Apple automatically shares internet over that carplay connection?

I have no doubt that the car will use the internet connection if one is exposed, I just doubt it will be exposed automatically.