xfce-base/xfce4-session always installs /etc/xdg/autostart/xscreensaver.desktop. Even when emerged with USE="-xscreensaver" and xscreensaver is not installed.
And it should, the USE flag is merely a convinience USE flag to pull in *some* runtime application that provides locker functionality for the xflock4 script. For autostart you should use the checkbox in Xfce's menu 'Settings > Application Autostart'... that's how you enable/disable XDG autostart items, not by 'not having the file installed'