@Bikeman You don't have to worry about that. The way it works is the charger/host tells the device which voltage levels it supports, and max amperage, then the device (your phone) tells the charger which voltage level it wants, and your phone will draw only up to the allowed amperage.
Of course anything can be buggy or broken, but if there was a major issue with the Tesla USB-C port outputting a different voltage than asked for by the default, you can bet we'd be seeing reports of issues here. That would be a very horrible and unusual bug for a USB-C / USB PD power supply.
The most common USB-C power issues involve connecting high powered devices to low-powered chargers, not the other way around. There's also occasionally buggy charger/host ports that will present live 5V on a USB-C receptacle, which they shouldn't (unlike USB-A), but an iPhone would handle that just fine anyways. (Danger would be if you connected that buggy port to 5V live on the other side, e.g. USB-A port or captive USB-C charger cable.)