I don't think it's particularly confusing here. Mantle is going to be a huge performance boost. It's Windows only. Ohh yeah, don't worry, it'll be cross-platform... some day. Just like some day AMD will release non-crappy drivers for Linux. And some day AMD will have a decent video playback solution for Linux. And some day AMD will have Android tablets. Some day.Huh? What does one have to do with the other? SteamOS is a longterm initiative. Maybe you're not seeing the helicopter view.
In any case I say this announcement does hold some good things, for one its lighting fires under complacent asses that haven't been innovating. Innovation in the PC space has been kind of sedentary and up until yesterday really only Valve has been interested in igniting new fires in this space.
In the meantime it'll be Windows only.
If this Mantle isn't cross-platform by the time SteamOS releases, I'll have no doubt that MS is pulling strings at AMD to keep this thing on their platform.