Are you sure the game is running on the correct GPU?. My older GTX1080 was always supported fine by proton. I’m not familiar with how garuda is set up but i assume it has a properly working PRIME setup by default. See if you have nvtop installed and on which GPU the game actually runs. For more info look here: https://wiki.archlinux.org/title/PRIME

If only it was mentioned in the article…

This is mainly data reported from desktop PCs, so no, SteamOS is not a thing at the moment on such machines.