Not the most important question since it has nothing to do with the fleet developer API.
But yes, if you are fine with not using Superchargers (which would be insane) or the built-in internet then you don't need to deal with the vendor after purchasing the car.
The whole point is that these services could be provided by other service-providers. This is like a car of brand X that only accepts fuel of brand X (except now it is a service like internet or an API).
You can use your own internet connection, the car connects to WiFi.
As for the API, then yes, if you want to buy a fleet of Teslas and manage them with your custom software, you need to go through Tesla. That API is a service and not a product. The car is still a product though.
But yes, if you are fine with not using Superchargers (which would be insane) or the built-in internet then you don't need to deal with the vendor after purchasing the car.