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.
It's based upon the well established distro Arch, and thus still considered Linux. A distro is basically the Linux kernel with pre-installed packages. SteamOS only adds another layer of packages unto Arch afaik.
The terminology is off then. Different distro's is not regarded as entirely new OS's, they're still Linux. E.g. SteamOS (if anything) is Steam's distro, not Steam's OS. I'm not trying to nitpick, only explain.
Even entertaining the idea it being anything else is ridiculous.
It's their own OS running on their own custom handheld. Treating it separately from other linux machines might be odd, but calling it "ridiculous" is being childish.
The Android and iOS apps don't actually run games, they're essentially just the store and community tabs + SteamGuard. The hardware survey explicitly exists to tally up what kinds of hardware is actually being used to play games on Steam, so that's why it's not counted.
I'd say it counts as general user base, but does not count towards the Steam survey.
If Android share counts towards Linux gaming in general, then I think "Linux" as a whole (not necessarily versions or derivatives of Linux that users have the choice to install on their device themselves, however) wins handily. That's because billions of people are all playing Clash of Legends of Heroes of Genshin Tower Puzzle Blocks Crash Deluxe, or whatever the fuck, on their Android phones.