And a window manager does the same thing even if you run KDE or GNOME. For instance if you run GNOME using sawfish as your WM, then kill sawfish, your GUI is seriously hampered. However start up enlightenment instead and you are back in business. (Things may look a little different...)
Conversely you can work perfectly well with programs that require either desktop environment from within a window manager that knows about neither.
The truth is that the final user interface has a lot of components behind it. I will grant that a desktop environment is part of the user interface that a given user may take for granted. But removal of that desktop does not mean that there is no graphical user interface left, it merely means that it behaves somewhat differently and you can't run some applications.
Cheers,
Ben