I wouldn't be surprised if Steam for Linux just created it's own chrooted environment and user account and then simply installed it's own set of libraries which would then be shared amongst steam programs but not with the system in general. X11 might be complicated of course (because Steam would have to run in the same X11 session as other software), but perhaps Valve are planning this around Wayland?

What this would provide is a "distro within a distro" where only things like the kernel version would vary. This would potentially allow anything within steam to sidestep a load of the distro compatibility issues. Instead of creating a .deb and a .rpm you just package for Steam.