That is HomeKit integration, which is somewhat different from third-party app integration. Siri works with HomeKit, HomeKit is an official Apple framework, and third parties can create HomeKit-enabled devices.
In theory, Tesla could make the Model S a HomeKit-enabled device and make it work that way, but you're limited to built-in device types and commands, so you'd have to call the car a "switch" or "garage door opener" or similar, and the commands would be "open" or "turn off." It would not be a good experience as it currently stands.