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.
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.