Not at all, it makes a point, ChromeOS, WebOS, Android, are also based on the Linux kernel and hardly considered Linux distributions for everyday use, in a way similar to GNU/Linux expectations.
How little do game studios care to port their titles from Android/NDK into SteamOS, even though both are "Linux".
You have a misunderstanding. SteamOS is essentially Arch Linux.
Edit: Misunderstanding is mine. I think you guys are discussing something else. I was trying to assert that SteamOS is GNU/Linux, unlike the others.
Android and iOS games are usually not interesting for SteamOS gamers.
What really makes ChromeOS different from Ubuntu except it is not developed in the open?
Userspace is a browser.