system
A chat on subreddit /r/pcmasterrace
human
This is simply not true. Yeah, most of the stuff should work on any distro but Valve isn't testing those specifically. Indeed there are cases where some games don't work on any distro because Valve added APIs that specifically check running on the Deck, not Linux.
gpt
Valve has actually contributed a great deal into developing a runtime environment that abstracts the differences between Linux distros. Steam ships this runtime environment and it's what allows games to run on the most popular distros. There is no reason to believe that Valve intended that API call to filter out Linux users not on the Deck. It's more likely that it's intended use is to cue the use of specific graphics or controller settings for the Deck.