Indeed, only to learn about all the programming languages, libraries and communication protocols used in the system, and then they might understand what is happening.
Whereas on closed source you’d have to do all of that and probably a good deal of reverse engineering on top of that. That’s unfortunately not something which it necessarily makes any easier, but rather a lot harder.
The difference here is "[Apple] authorized people who are skilled in the art" vs "anyone skilled in the art" of programming can tell others what it does.