SteamOS is a linux distro, similar to any other. It’s a amalgamation of different pieces of software, including a desktop environment (plasma). But it does not boot into the desktop mode by default, instead it boots into their own graphical environment (gamemode) by default, running their steam client.

That’s because their main focus is gaming machines, and that’s why they want gamers to be greeted with a consolized, 10-feet UI.

I think your confused because you think of steamOS being the UI that welcomes you when you boot into it, instead steamOS is the entire package, including the desktop environment (which is KDE Plasma), gamemode, etc.