You can actually switch to desktop mode, which is a full KDE environment, so it is a Linux desktop PC.
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.
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.
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.
Ordinary laptops don't have these problems.