There is more to an 'OS' than the kernel.
SteamDecks are indistinguishable from PCs, since they run more or less exactly the same software as a Linux desktop/laptop would.
SteamDecks are indistinguishable from PCs, since they run more or less exactly the same software as a Linux desktop/laptop would.
At best it would be a general purpose mobile device, but even that is rather contrived. How many users do you imagine are using a steam deck as a substitute for a desktop or a different mobile device? I would guess something very close to 0.
Also, the Steam Deck's OS is by far the closest you can get to a traditional Linux distro since it's GNU+Linux under the hood. The built-in desktop mode is extremely close to a basic KDE Arch install, especially after you disable read-only mode. Android doesn't have as full-featured of a desktop experience unless we count Samsung DeX and even then display out is available on a vast minority of devices. iOS has no native display out and iPadOS doesn't even support normal 16:9 screens without black borders. The issue with all of the above devices is that their "desktop" modes are janky afterthoughts while on the Steam Deck it's a core feature.
Also, the deck does not replicate a subset of the desktop experience, it just contains a desktop experience. Unless the KDE desktop is not a desktop now. If that's the case - it runs windows.
But Steam Deck is just running a fork of Arch not some other operating system. From analytics perspective it's indistinguishable to any Linux PC. Also IIRC you can just connect it a display which would turn into a desktop PC.
Is it contrived? They've shown, in their official marketing videos, demonstrations of the steam deck being hooked up to an external monitor and being used to run KDE and Windows 10.