Usually the screensaver is an executable that draws stuff onto the screen. It is called by the screensaver program at the appropriate moments and run on the root screen. I don't remember if KDE screensaver had an option in it's config to add new screensavers...(?)
In reference to the initial post... though I only have gnome in front of me... I see <screensaver name>.desktop files just like you, also non-executable. But I did a hunt for all files relating to a particular screensaver - moire - and got this:
$ locate moire
As you see, the executable is in /usr/libexec/xscreensaver/ ... with all the others. I suspect a similar location for KDE.
Investigating the .desktop files shows they are text files ... the content is fairly self explainatory.
Enjoy your investigation. I too have had no luck finding a specific tutorial or howto about creating screensavers for KDE ... though they must exist. You could try asking the screensaver authors at KDE-look.org?