Are you sure about that? Why wouldn't this be more similar to Android than, say, Linux Mint? Who says they're even using any of the typical userspace bits?
I would bet a money (albeit a small amount) that, to a first approximation, they're really only using the Linux kernel.
As others have said, the important point here is that the core of Linux support is here. They've already ported all of their major titles to Ubuntu-based desktop linux, and I would be extremely surprised if SteamOS did not inherit greatly from that effort.
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 release of Steam on Ubuntu was a precursor to this. Valve encouraged developers to port their games to Ubuntu and ported their own games. It is unlikely that they would do that again. This will likely be a linux system with gnu userspace for now, even though they would have a custom UI.
Because they already ported Steam to Linux and have about 200 titles native on Linux. That fits quite well with "Hundreds of great games are already running natively on SteamOS." statement. I do not see why they would force game developers to do the work again.
I would bet a money (albeit a small amount) that, to a first approximation, they're really only using the Linux kernel.