It seems a tweet from Michael Marks the development director at porting house Aspyr Media has been taken out-of-context by another well known Linux website, time to clear things up.
Mantle is the new API from AMD developed as an alternative to OpenGL and Direct3D, for those that didn't know.
Here's the tweet in question:
Now, bear in mind this is specifically talking about the new Civilization game that is coming to Linux, it cannot support Mantle on Linux/SteamOS since Mantle isn't yet on Linux.
This will also help clear it up:
That tweet is directly from Nick Thibieroz the Gaming Engineering Manager for AMD.
I hope this clears any confusion up. Mantle isn't even that big of a deal anyway, since it currently will only support AMD hardware which has a bad history on Linux, and AMD, Intel & Nvidia all talked about unlocking 15x more performance with OpenGL anyway, so Mantle seems like a wasted effort.