xdg-screensaver does not recognize xfce4-screensaver
xfce4-screensaver should be used, if the daemon is running.
This test seems to detect it:
dbus-send --print-reply --dest=org.freedesktop.DBus /org/freedesktop/DBus org.freedesktop.DBus.GetNameOwner string:org.xfce.ScreenSaver