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.)