AFAIK they call it Plasma (just Plasma, not Plasma Desktop) because that is now the name for what was previously called KDE and KDE now is the name of the commu

AFAIK they call it Plasma (just Plasma, not Plasma Desktop) because that is now the name for what was previously called KDE and KDE now is the name of the community and project that acts as an umbrella for various other projects, including Plasma but also applications such as Krita, KDevelop, Amarok, etc. The desktop environment isn't officially called KDE anymore, it is Plasma.