People buying Steam decks are likely the majority of those numbers.

The Steam Deck shows up as Arch Linux in the steam numbers; Arch is only 10.7% of the Linux user-base on Steam. And this is on top of the fact desktop Arch Linux is a thing as well sharing space in that line-item.

The Steam Deck (and Microsoft tomfoolery) certainly was the catalyst for the current wave of adoption, but it is barely a notable percentage of Linux installs on Steam.