I did say "arguably".
I mean more in the sense of the hardware it uses, i.e. it's PC-compatible (x86) hardware that runs a desktop linux distribution by default.
The Steam Deck is literally a PC, no doubts about that. It's even more "P" than many other "C"s, like most Apple hardware for instance. But I think that the notion that it somehow validates Wayland as mature tech is kinda weak.
It appears that the justification for switching to Wayland in many distros (just like systemd before it) was so that the devs could actually abstract away work on these components and invest time into their flagship features (WM, filesystems, shells, look & feel, the works). For what its worth, looks like this was achieved.
At least it's closest to a "Linux distro" software-wise.
(Otherwise Android phones would be the most widespread Linux devices with a powerful GUI.)
While I wouldn't call Android phones "personal computers" either, they're much closer to being the "most popular Linux PC" than a Steam Deck is.