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.