SteamOS is highly customized and managed by Valve. It doesn't have the Arch repos or pacman. Updates to the OS come from Valve in large releases rather than per package. All apps installed either come through Steam or Flatpak.
So it's Arch in the same way Android could be called Linux.
The way SteamOS works is a bit weird. By default the OS partition is read only, you can do whatever in your user directory. But they let you unlock it to make whatever changes you want.
I’m pretty sure that this OS partition gets completely wiped every update though so your customisations will have to be reapplied.
The hardware is just an x86 pc. You can install windows or any other Linux distro on it.
SteamOS itself is designed as a hands off just works forever experience. You can tinker a surprising amount but it’s a very non standard configuration.
So it's Arch in the same way Android could be called Linux.