Say Team A within Apple creates a module that is only to be used by Team B and C. Then Teams D creates a module that depends on Team A and B and they release a public API.
Team A should publish an internal documentation and Teams B and C should also not use methods that were not documented by Team A and Team A shouldn’t follow standard semantic versioning. If Teams B and C need some functionality they should inform Team A.
The same goes for Teams B and C. Also, Apple controls the release schedule for any internal teams. If they want to move to ARM tomorrow they can coordinate it within Apple without breaking any third party apps. The only team that has any responsibility to not break external developers is Team D.
If Apple starts allowing external developers to use private functions now their release schedule is tied to when enough major third party developers update their documentation.
Amazon is even a better example. They have a policy that everything should be a service internally and no team should reach “inside” of the implementation of another team.
I don't know what Apple means by "secret APIs", but I'm assuming they mean "undocumented APIs". Those are certainly real things that commonly exist.
> By definition an Application Programmers Interface is the vendors promise to other developers that when you call it
Yes, but those "other developers" may be other engineers in the same company. The APIs they use are still APIs, but are not intended for use by anyone outside the company.