XBox One is GCN also. Seems if M$ was to port the console API to Windows it would have to be as GCN specific as Mantle is until others adopted the minimum feature set, which they might already support, and joined on.
Here's a consideration. What if M$ couldn't adapt DX to what the Devs. wanted? Possibly because they couldn't break backward compatibility. AMD has said that there was a dialog with M$ concerning Mantle. The quote was something to the effect that they wouldn't do anything to damage the relationship they have with M$. They aren't blindsiding M$ with Mantle. M$ could have needed a way out of DX moving forward and Mantle and the Xbox1 API could be a means of doing it?
What's up with the "M$" stuff? Is it 1999 again? Is that still cool or something? Like it or not Win7/8 and DirectX are by far the biggest gaming platforms on the PC and are, and will remain, ubiquitous for PC gaming. Even titles that potentially have Mantle support such as BF4, DX11 support will be there as well. Conversely, while I don't "dislike" OGL, there has been like, 1 openGL title of note in the past 5 years (non indie title) and that one was a bust - Rage. Obviously DX is and always will be the biggest PC gaming platform and MS is taking steps to improve it.
This hyperbole about Mantle "killing" DX is just ridiculous because we have Mantle here that might have 1 title in December. (Not you in particular, but others have stated this, and it is ridiculous) Meanwhile, everything is on DX and it will remain that way for years. Not sure what your issues with the whole "M$" thing is either, it isn't 1999 anymore. Now, all that said, I do welcome Mantle and i'm eager to see what it brings in terms of performance particularly with BF4. It could be great. We'll just have to wait and see. But Mantle is going to be a game changer because the biggest as of yet multiplatform engines will not be supporting Mantle directly unless a developer rewrites the engine from scratch (ie Unreal Engine 4).