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Arch takes the majority because SteamOS is based on it. Unfortunately, I don't think there's anything in the data that would allow discerning between those two.
From the comments on the video the uploader said that they use ProtonDB which separates steam deck from normal Arch
I think SteamOS doesn't report itself as Arch, but as SteamOS/Holo.
I'm not sure if that's the case, since looking at the stats you'd expect SteamOS/Holo to beat a couple of these listings if it was truly being counted separately to Arch
ProtonDB reports PC and Steam Deck separately. So unless the video creator took the PC stats and then added in Steam Deck and just counted SteamOS as Arch themselves, it's probably just omitted entirely.
Also actually watching the video: Arch was already riding along at 20-21% before 2022. And in some cases dipped down to 19% mid 2022... So after the Steam Deck launched, the Arch numbers went DOWN. So yet another sign that Arch isn't Arch + SteamOS: this is purely desktop usage.