I had a discussion about this 1 months ago: beehaw.org/post/14768525 And decided not to bring it up again. :D
My argument is, from the eyes of the website you visit, the Steam Deck user would be identified as a desktop user. That’s because the browser you are using (most likely Firefox) and the desktop environment (most likely KDE) in the Desktop mode would be seen as a desktop. In short, yes, I think Steam Deck would be counted, but only if people visit the pages in Desktop mode. So not all Steam Deck users are counted here.