Ok just for fun I just checked "src/gui/styles/qgtkpainter.cpp" in Qt 4.7.3. It really calls gtk to do the drawing (gdk_pixbuf, gtk_icon, GtkWidget). I haven't checked for the other backends, but wouldn't know why this would be different. You can also check by changing the GTK theme. If it drew fake gtk-ish controls it would ignore that.
Look harder. Its drawing its own emulated versions of the controls. It is re-implementing the behavior and it is emulating the look of each control by drawing its own stuff into *generic empty* windows. It is using some native graphics and theming API to draw its own things that look like the original native controls.