In the announcement page, it's mentioned only SteamOS, not Linux. In fact, it's present only the logo of the Valve's OS, not Tux.
Are we sure that will be compatible with Linux?
The way for developers to release their games for SteamOS is to release them on Steam for Linux. SteamOS is not a different category in Steam, it's just a few more restrictions on top of Steam, e.g. using the Steam Runtime (SDL2) etc., so yes, it will run everywhere thanks to the Steam Runtime.
Also this is actually huge news, this is a serious AAA racing game.
Also, Valve has just announced that SteamBox testers will get Metro Last Light and some other games to be announced in the next days for free.
Although Android is Linux. Yet it seems to me that what runs on Android can run on Ubuntu, no?
SteamOS isn't based on Ubuntu, as written on Engadged. Our hope is that it's a GNU/Linux custom distro, and not a simply custom OS based on Linux kernel, like Android
What is this Steam Runtime people are talking about? Is that a layer on top of OpenGL? How is that different from SDL? Is it part of the Steam client already available on Linux?
It's really just a collection of all the libraries that steam and it's games need to run. I don't know for sure if it is statically compiled or not, but it includes all the dependencies that steam needs. including SDL2 etc. You don't need the runtime as long as your distro's package manager installs all of steams dependencies.