To add this overlay from layman, do:
add to /etc/layman/layman.cfg
overlays : https://raw.githubusercontent.com/hardentoo/hardentoo/master/overlay.xml
then
sudo layman -a hardentoo
or run:
sudo su
layman -f -o https://raw.githubusercontent.com/hardentoo/hardentoo/master/overlay.xml -a hardentoo