They mean that most people don't think of Steam Deck users as "Linux Gamers". SteamOS is essentially an appliance OS. It's what comes with the Steam Deck. People don't choose it, it's the default.
The only reason I care about the number of Linux users is because more users means more development resources. Using that criteria, Steam Deck users very much “count”.
That's technically true, but oversimplified enough to be misleading. SteamOS boots from a read-only OS partition from which you can change nothing. You can only write to your "home folder", and you can only add new software using Flatpak, or Steam itself.