I would bet a money (albeit a small amount) that, to a first approximation, they're really only using the Linux kernel.
Even if SteamOS uses different toolkits or packaging (which it very well might), the amount of effort to get those titles to run on typical desktop linux distros should be relatively very minimal.
I highly doubt it would be more similar to Android than Linux Mint simply because they have already invested time and money into making Steam run on the Linux Mint family of distros (i.e Ubuntu), and (to my knowledge) none working on the android stack.
The rest is irrelevant; if a game runs on the SteamOS, it will run on my Linux desktop.
When we actually see it, it may be either a desktop-less distro, have Unity/GNOME or some other desktop permutation. My best estimate of what best fits a maximum impact strategy is an OS with a desktop environment pre-installed (even if hidden from casual view) or easily installable.
I have a feeling that we may look back at this seemingly minor product announcement as a significant stepping stone.
Maybe with added Wine.
Nothing in the release said steam only or no desktop, and they call out the ability to create content on the platform specifically Users can alter or replace any part of the software or hardware they want. Gamers are empowered to join in the creation of the games they love. Rather sounds like what they are doing is going to be open source and allow non games to run. Even if they don't ship with KDE or Gnome by default it should be possible to get something up and running.