I would say, yes gnome-flashback can work without screensaver, but
1)
Lock Screen applet will not work without gnome-screensaver. I think this is only thing that directly requires gnome-screensaver. Lock Screen applet requires gnome-screensaver D-BUS service.
2) Screen locking will not work as said in quoted text. But I guess this is not problem. gnome-screensaver is installed by default in ubuntu and it can be removed (it is not a dependency for any other package in ubuntu). But removing it lock does not work in ubuntu session too.