The top distro is Arch - implying that the Steam Deck userbase is moving the needle. Linus has said on a few occasions that the main thing holding back user adoption for desktop is a single distro with a clear focus. What Android did for mobile. It's clear that SteamOS could be "that guy" if Valve wants it to be.
But yes, SteamOS makes ~25% of the users. Though, thinking about, do they collect per account, or per device? I do have a Steamdeck, but mainly play on the big desktop running on debian, so I'm curious if I'm appearing as one or two entries in that stat.