That seems really unlikely. Steam for Linux runs on Ubuntu as it's platform of choice, and installs via apt-get. They're already running all their games under X and the standard Ubuntu userspace tools, with BigPicture, it would be very surprising for them to fork the platform at this point.
Android made sense because standard linux/X windows applications didn't work on a multi-touch mobile device for a whole host of reasons. PC games work fine on Linux and X.
X has a lot of baggage. And by lot, I mean a few metric tons. The protocol can do things you wouldn't believe, and do that in the most cumbersome way (it was all the range in the Eighties, or so I'm told).
Most of Steam and Steam games don't really use X as anything but a bootstrap for GL, and run full screen, so they could be easily ported to FB+GL (consider that there's already a GTK backend that renders to FB, no X involved).
Android made sense because standard linux/X windows applications didn't work on a multi-touch mobile device for a whole host of reasons. PC games work fine on Linux and X.