Without getting too much in the middle of these, I think you underestimate Tesla's ability to push out updates to it's cars...they've designed them that way. Even for something as complicated as CarPlay.
As long as Tesla has done the work on their end to make sure CarPlay integration works with their systems, I believe they could push out an update to add it to their vehicles.
Tesla is way more advanced and adaptable when it comes to this kind of stuff where most large car companies require you to visit a dealer to apply an update to their system...and then it would probably not work as well...
Tesla has very limited models running a very similar software/hardware for every car...they could do it.