There's such a deep seeded, systemic bias against linux that it actually can never win, to any degree or magnitude, because the moment it starts winning we just move the goal-posts for the flimsiest of reasons to ensure it can't quite claim that victory.
Linux is obviously and clearly the most popular operating system kernel on the planet. Oh, no, that's no good a measure, servers are messy, let's refine it to most popular consumer operating system kernel? Oh... it, could also reasonably claim that title? No no, no Android, that doesn't count. Nope, No Chrome OS either, you can't have that, that's, well, that is linux, but its not. Just nice, pure, desktop linux, yes, perfect, arch linux, kde desktop, that'll never trend up and thus is the perfect new-new definition of desktop linu--wait hold up, I'm getting word this is, not possible, its actually SteamOS? Nope, kill it, that's not desktop linux either, kill it.
So it is proper Linux, as GP comment implies. Yes it's running games in Windows compatibility layers, but it is also a complete Linux system itself, with desktop. Definitely counts as running Linux.
And a decent chunk of those games are running on the Unity or Unreal runtimes. Do they count as "running on Windows"? Where are we drawing the line here?
this is fascinating..
Is it possible to install Arch linux and add the SteamOS layer and cut over and back again as desired?
You can also just go to the normal KDE Plasma environment from the steam menu and use it like normal Linux - or even install let's say emulators from Arch repos and add them to Steam then run them from SteamOS interface.
I've only run SteamOS on my Deck, but afaik if you install it on something else then you can still flip it between the gaming mode and the desktop mode. In desktop mode it behaves exactly like you'd expect an Arch Linux install to behave, and you can mess around with it as much as you'd like. In gaming mode it's like a console and really only plays games (but isn't limited to Steam games - people have got it running emulators and all sorts of other stuff too).