I'm more confused that it's running SteamOS which is supposedly Arch based, but arch doesn't officially support ARM. You have to use the ArchLinuxARM distro for that, which is less maintained. They got to be doing something off label for that.
Arch has been working with Valve on various build system improvements for some time [0], which as I understand it are targeted at making it more feasible for them to eventually support more architectures [1]. This doesn't release for several months; I wonder if there'll be an official Arch Linux ARM by then?