Earlier quoted context omitted.
Which shows how far ahead KDE was even in 3.5 times with reusable embeddable components that were highly engineered. For example Konqueror has a long list of plugins including calling kwrite to render inside of it. The early kio tooling that predates fuse in having universal access through a single file viewer.
And it is so frustrating to see how unknown and neglected KParts is
Also while it might not be known to users (why would it need to be), it is used in various KDE programs where it makes sense.