Quote Originally Posted by Marishi-Ten View Post
Mac runs off a custom Linux kernel that is closed off
It's based off FreeBSD not Linux (for the record PS3 and PS4 OS are also based off FreeBSD)

Realistically its down to how well they've realized their graphical engine abstraction, which considering the game runs on multiple OS has been done to some extent. I'd wager porting to OpenGL should be doable for them. Valve has proven lately that gaming on OpenGL on Unix systems can be more optimized than their DX/Windows counterparts.