It is plain wrong. Most KDE applications are written in QWidgets.
Seems like some use XML to compose the UI?
Qt Designer .ui files I guess, these are used to generate C++ (QWidgets) code that isn't substantially worse than hand-written. There is also KXmlGui, but that is a rather particular thing IIRC mostly about merging menus from plugins and about permissions.
You are free to write one, and it works. I have a few internal tools that do not touch QML or the XML ui system, and it still builds just fine with then latest libraries on Arch.
Of course an upstream KDE application might not be written that way, if that's what you are complaining about.