A desktop PC is not a type of operating system. It’s a type of computer designed for a specific use case, distinct from servers, mobile devices, handheld gaming devices or gaming consoles. A windows server or Linux server with KDE/gnome is not a desktop. A mobile phone isn’t a desktop, and it wouldn’t become a desktop even if you installed a plain Linux distro with KDE on it. A PlayStation, switch or Xbox isn’t a desktop PC, and neither is a steamdeck.
It’s an arbitrarily defined category of computer, and this statistics site doesn’t mention how they’ve specified that definition, but it seems they haven’t included any server, mobile/handheld, or gaming console devices at all, not just the steamdeck.
It really sounds like you don't understand how the OS is set up on the steam deck. It's a normal desktop environment with a customized version of steam installed.
If laptops get to be included (which they usually are), then steamdeck gets to be included.
It doesn't replicate "some subset" of the desktop experience. It does everything a small tower can, plus things it can't do.
Steam Deck's desktop environment is barely usable without connecting it to a dock and using an external keyboard and mouse.
You won't use it as a daily driver in handheld mode. The virtual keyboard covers half of the screen, the touchscreen is unusable as a mouse and touchpads are inconvenient.
I deliberately didn't compare it to a laptop, because the topic at hand is whether it qualifies as a "desktop". If it can function without an external keyboard and mouse that's a bonus feature.
It’s an arbitrarily defined category of computer, and this statistics site doesn’t mention how they’ve specified that definition, but it seems they haven’t included any server, mobile/handheld, or gaming console devices at all, not just the steamdeck.