

SteamOS seems to not be counted at all in the first page. Apparently, it’s not just “All OSs combined” vs “Linux only” but there are additional filters applied. Perhaps the first page is desktop-only. The article either also cares about desktop gaming specifically or is uncritically parroting the survey page. I think both Valve and the article writer should be clearer about what they’re talking about.
Yes, Proton is primarily a Steam thing, but it’s free software and it’s being actively ported to other launchers as umu (not by Valve). The project is very new and I’m not sure it perfectly matches Proton behavior yet, at least as far as game-specific tweaks are concerned.
Personally, for a non-Steam game I would just try to run it via Lutris. Lutris tries to automatically setup everything so you don’t need to tinker with anything in the best case. It even automatically downloads the game installer and wine, and you can configure it to use the aforementioned umu instead of vanilla wine. In the ideal case, you get the game installed and running with minimal effort all from within the Lutris client. The problem is that the Lutris scripts are maintained by the project itself with recommended corrections from the community. So it’s possible that a game could run with tinkering, but it hasn’t been automated yet.