The article claims it's because Arch is "lightweight," but then why not pick a smaller distro?
My best guess would be familiarity (Valve can afford to buy the best developers, and lots of great folks enjoy the fiddlyness of Arch), hackability, and insanely fantastic documentation of Arch
So I wonder why not NixOS since it's safe updates (an update essentially can't break anything, not necessarily true for database etc outside Nix but still much saver than any other non-declarative distro) and effortless rollbacks, customization etc. It's community is problematic right now, but it's still the most complete modern distro we have, with Guix due to the GNU design a bit behind and all the others FAR behind.
It means a fix in graphic stack or proton will either,
1. deliver after a whole year, and get actually feedback after two years.
2. require a lot of backport so steam can actually use it.
Rolling update in the other end, suits well with the changing nature of graphic stacks.
I have wine/proton on FreeBSD and FreeBSD has also a Linuxlator (a Linux system call translator), so no blocker here.
But you are right, if you want a open-platform (opposed to Playstation), Linux is probably the better choice atm.
The steamdeck runs an AMD chip, which unlike nvidia, is an official first-party open source graphics driver with a decade of development behind it. It works for literally every game I've ever thrown at it. FreeBSD also runs the same open source driver.
I'd be interested to see a comparison in performance of the same (recent) game on the same hardware with Windows vs. Linux vs. FreeBSD; I assume there will be a small penalty from Windows to Linux (in general) and a larger one from Windows to FreeBSD, just because the games are not optimized for the best way to do things on the other platforms. Syscall emulation gets you comparable functionality but not necessarily comparable performance, given there's mismatch between the underlying abstractions.
On the driver front, even if it's just nVidia that's holding things back [edit: apparently they're not], that's still going to affect most PC gamers.
https://www.nvidia.com/en-us/drivers/unix/ lists official drivers for FreeBSD and Solaris (which also apparently are used by illumos). In fact, I suspect they have a better time than Linux on account of having actual stable driver ABIs.
I will not accept ill talk of the Steam Link slander /s
I got mine for 6€ new when they stopped production. Don't use it much, but its nice to have that when I have a system no where close to an actual TV.