SteamOS 2.x was based on debian, but that hasn’t been updated in years. The Steam Deck launched with SteamOS 3, which is actually built on top of arch and is much more akin to Manjaro. As for your question, it’s mostly the “game mode”, which uses IIRC Wayland and wraps games inside gamescope which provides a bit more control in the form of controlling frame rate, resolution, etc externally, but regardless, that can and has been achieved in custom distros. I think the main appeal of SteamOS honestly is the package of an immutable OS optimized for running games on steam. It prevents non-linux users from breaking things and tries to make it feel more like a “console” with a “desktop mode” (KDE Plasma) and “app store” (ala flatpaks). I’ve toyed with the idea of running it or similar on my gaming PC but always run into the difficulty of Nvidia drivers on Linux.
Funny you mention the devs… Not my project, but at least one of us is here