It's not. Apple's only certified for "macOS version 10.12 Sierra on Intel-based Mac computers" (previously that was "Mac OS X Version 10.5 Leopard on Intel-based Macintosh computers", etc)
I don't think iOS is anything else than OS X with a different facade (Cocoa Touch, which for UNIX doesn't matter) and a few additional services. It has all the usual POSIX interfaces (and some of the userland) underneath. So even if it violates some models (e.g. with regards to user accounts), it's still the same codebase, heralding from the BSD years.
