Yes, SteamOS does count as Linux. Android does not. The Android and iOS Steam app is just for social features / store, not for playing games so neither show up on the survey.
SteamOS Holo, which is what the Steam Deck uses, makes up 42% of the Linux systems in the survey results.
This was the first thing I thought of when I saw the title of the post. Mindustry is awesome, I got so addicted to it for a bit haha