I have to say that this is nothing short of a miracle. The OpenGL API is really hairy to implement at all, let alone making it fast. There's 20 years of accumulated legacy and design mistakes that the driver vendors need to care about. D3D, on the other hand has been re-designed on every iteration and backwards compatibility has been broken in every major release.
On the other hand, porting from D3D code, you don't touch the hairy parts of OpenGL at all because D3D is functionally quite close to the "clean parts" of GL. But there's still some software overhead to overcome. They might also be using a proprietary D3D-on-OpenGL layer, like the ones being used for some Mac ports of Windows games (edit: seems like they are not).
I hope that Steam, and gamers that use Mac and Linux in general, will push for bigger changes in OpenGL, both specs and implementations.
On the other hand, porting from D3D code, you don't touch the hairy parts of OpenGL at all because D3D is functionally quite close to the "clean parts" of GL. But there's still some software overhead to overcome. They might also be using a proprietary D3D-on-OpenGL layer, like the ones being used for some Mac ports of Windows games (edit: seems like they are not).
I hope that Steam, and gamers that use Mac and Linux in general, will push for bigger changes in OpenGL, both specs and implementations.
disclaimer: I write GPU drivers for living