Desktops can be installed on any ubuntu release. you could install xfce on kubuntu, or kde on base ubuntu. And what DE/WM you are using doesn't matter in a quite a few scenarios, so it's not worth bringing up in the same way that I don't mention "Im using cachyOS niri wm"

I personally preferred xubuntu over kubuntu back in my *ubuntu days. etc etc. there are a lot of spins of ubuntu and when the only differentiator is DE (which you can also just install on base ubuntu), it's easier to just talk about them all as a single overaching entity/family ... ubuntu.

Talking about kubuntu inherently requires talking about KDE and ubuntu both, so people just skip the KDE discussion.

"Too many distro options" is already a complaint of a lot of non-linux users, so specifying every last spin can just drive people away.