No you are not. Or you weren't. Allow me to quote your own post:
I’m talking about the “DE” part of KDE in general;
As the DE is Plasma, that is the part I am addressing. Now you are moving the goalposts. That said, I do not know what you mean when you refer to "the KDE project", as KDE encompasses many projects.
In any case, I don’t doubt that KDE can’t run at all under the specs you mentioned
So you don't doubt it is light. Of course if we pile on a bunch of apps, like we could throw in Blender open 50 times rendering 4K animations and I'm sure it will make the laptop run slow. But that would be because of Blender, not the DE.
However, for the sake of argument, I did try the three examples you quoted, Dolphin, Konsole and Kate, and as you can see in the aforementioned video, they are all also very light and worked perfectly simultaneously on the 2008 machine. I do not have Konqueror installed on that machine, as it is not considered an essential part of Plasma anymore and is not widely used.