By itself xfce-extra/xfce4-screensaver only has 4 screensavers, the rest should be provided by x11-misc/xscreensaver.
For xfce-extra/xfce4-screensaver to see these a per-screensaver .desktop file in /usr/share/applications/screensavers is needed.
The format of the required files is documented here:
These can be easily auto-generated, usually is just the path to the screensaver (or "hack" in xscreensaver terminology) followed by the "-root" command line argument.
Remaining command line arguments are automatically read from screensaver's XML description file that's already part of the x11-misc/xscreensaver package.
Sounds more like an optfeature thing?
It could be an optfeature, however then it wouldn't be clear which package should own /usr/share/applications/screensavers/*.desktop files.
Making it an USE flag instead has an additional benefit of making the installed x11-misc/xscreensaver package automatically eligible for depclean if xfce-extra/xfce4-screensaver is removed.