Interestingly enough, you don't need Apple's support to do that: https://github.com/st3fan/dovecot-xaps-daemon.

Apple didn't have to build an API for them, they simply provided documentation for how to use the existing API.

Sure, and occasionally the add stuff and consult with Mac software houses and indie devs (or musicians and editors for their Pro stuff) on what to add etc.

But that's not the same as building a bunch of APIs for a competitor.