Because the OP here was actually mistaking things. For all we know there won't be a Tesla to CCS adapter that works on existing Ford/GM vehicles, it could be that only Ford/GM vehicles with the NACS connector will get to use all superchargers.
And Tesla has already opened up charging to all EVs on some superchargers via the Magic Dock, but the situation isn't amazing, especially with many new cars using an 800 volt architecture and thus not charging super fast on Tesla's 400 volt superchargers. https://youtu.be/pUYqDNCIFD8
Combine this with how many charge port doors aren't on the back left or front right, and you have a really messy charging situation with blocked spots and slow charging cars.
The partnership most likely means:
- The cars will be 400 volt architecture (or otherwise 800v and upcoming V4 superchargers will support up to 1000 volts)
- The charging port door will be either on the front right quarter of the car, or back left quarter.