"Mantle guts the overhead for cross-platform development by a huge amount, meaning you can develop something on one platform and release it on all others in little to no time."
You mean exactly what DirectX already does? And OpenGL? Oh.
One developer and one engine adopting Mantle doesn't prove any sort of mass acceptance.
If anything I see one company (ATI) that is horrible at drivers, teaming up with another company that has had nothing but stability and performance problems with their new engine... teaming up to find out that "hey... with some better programming our stuff wouldn't be so terribly inefficient like it is today".
Nothing about this announcement says that nVidia wouldn't see the exact same performance gains either.