I have /usr/share/tvheadend/src/webui/static. Those are all old files! 
The downloaded repository contains the new files, but they are not copied over. As make never does.
So a sudo make install is required, wonder why README.md does not include that instruction. Surely I have done is at least once before I started following the README instructions.
A sudo systemctl restart tvheadend is needed so the new webui is picked up.
The binary is now copied to /usr/local/bin so I have to take care of that. Change my startup script that is.
All works now.