It might be something like this?

c. Nobody else who licenses ARM would be allowed to add extensions to their implementation of the ISA in this way. Apple is more or less a founding member of ARM and has some other deal.

Yes, though that's a different formulation of (2) really:

> Nobody else but Apple has the right to have these ISA extensions

Programmers "can" use Apple extensions because... they're extensions to the ISA, they're not documented but they can't really be blocked off unless they're ring0, and that would make them a lot less useful (the entire point of having extensions for matrix computation is to speed up matrix computations, which are not usually kernel work, and having to syscall to do them would waste the entire thing).

Apple exposes these behind APIs that hide these implementation details. Their own compilers never emit them. Nothing can stop you from embedding them into your binary yourself, of course, but this would be annoying to both you and Apple.