As a KDE developer: That's incorrect. While the Plasma desktop shell does support widgets "in the web run-time sense" (it has support for Apple Dashboard widgets, which are written in HTML/CSS and JavaScript), the video in question shows native Plasma widgets, which are not web documents (i.e. UI elements, layout management, etc., are not done by a web content engine, but with Qt technology). They're written in C++, JavaScript, Python or other languages bindings exist for. Of course, being Qt, it's trivial to throw a WebKit element into a Plasma widget and show some web content in it, however.

Plasma requires KDE's libraries to run in addition to Qt, and various things shown in the video make use of technology found in those libraries - the freedesktop.org icon naming spec-compliant icon loading and the implementation of the calendar system, for example. In fact, Plasma's own library (which is also used by applications other than Plasma, for example the popular music player/manager Amarok) is part of kdelibs as well.

If you take a look at the info box for the video on YouTube, you will find a link to the sources in KDE's SVN repository there, including the S60 port of kdelibs. It's not a complete kdelibs, but I wouldn't really characterize it - or Plasma's dependencies on kdelibs - as minimal.