I disagree. I don’t believe that distinction is relevant. My reasoning is that the large number of SteamOS users allows Valve to justify spending time and resources improving things like Mesa and amdgpu (edit: and of course Proton, which is huge), which benefits many Linux gamers that do not own Steam Deck. I use the SteamOS compositor (gamescope) on my desktop PC because it’s great—-and it probably wouldn’t exist without SteamOS.

The only reason I care about the number of Linux users is because more users means more development resources. Using that criteria, Steam Deck users very much “count”.

SteamOS is just Arch. You can use it for anything you could use Arch for. The fact that people elect to mainly use it for gaming doesn't really mean that's all it's designed for. I also have a gaming PC that runs Windows 11 and I never use it for anything other than gaming. Doesn't mean it couldn't be used for other things.