Well, now I understand what a Desktop computer is. But it doesn’t change the fact that the Steam Deck can be one in docked mode.
Just because the default OS isn’t what you have on your daily machine and also doesn’t have this cool “Media mode” some people refer to Gaming mode, doesn’t make it less of a Hardware it already is. Just install Windows of it and stop removed about it.
You first said a “Desktop” is only a tower PC or rather a Desktop PC that sits at the desk. But then you gave a scenario where you complained that a Desktop mode didn’t work.
You do notice that Desktop Metaphor and Desktop PC are two entirely different things. Generally clients can be referred as Desktop to make things easy.
A Laptop and Handheld can be described as Desktops too.