I’m sure that’s available somewhere too; it’s not as if Valve is massively violating the GPL or something. (If they were, it would’ve been big news by now.)
SteamOS as a whole is not open source. Most of it is, but it also includes proprietary software (e.g. Steam itself). This is likely why you were downvoted, as SteamOS can be kept private without violating any license thus your first statement was false.
Valve could distribute each single piece of open source software they use on request to their customers, without publishing any guide to actually build it. (Thanks for linking to Valve’s repo, which seems to match this statement.)
This is how Apple does it with Darwin, the BSD-derived open source core of macOS. Without all the proprietary parts it’s not useful as an OS, even though they follow all the necessary licensing.
