There's nothing stopping you from using an OpenGL api that's built on top of DirectX, though. What Apple has done here is say that you can't put an API between you and the underlying API such that you are insulated to some degree from platform changes; but even worse, they've mandated which languages (!!!) you can use to interop with that API, which is almost criminal, technically speaking. (Of course it's not de jure criminal. Just for the reading comprehension impaired.)
Microsoft did the same thing with DirectX and OpenGL. OpenGL was cross-platform. Microsoft stopped supporting it, and instead insisted that developers use DirectX. This essentially killed cross-platform gaming for Mac/Linux. DirectX games port between Microsoft platforms and Microsoft's Console, but require significant middleware to port to OpenGL and non-Win32 platforms.
Use of an API, public and documented, is a form of dependence and lock-in. Apple has done. Microsoft does it. Oracle does it.