The post-install creates user & group 'xbmc', but the installed systemd service expects user 'kodi'. The post-install needs to be updated to create user / group 'kodi' instead.

After installation, I had to do this to fix it:

sudo usermod -l kodi -d /var/lib/kodi -c "Kodi user" xbmc 
sudo groupmod -n kodi xbmc

See also the AUR package:
https://aur.archlinux.org/cgit/aur.git/tree/readme.install?h=kodi-standalone-service