Confirmed with Kodi, I tried a few methods with no success, doesnt seem to like being launched under bash with systemD.
Needs further investigations, will need to pick this up at a later date.
Simple solution for now is using your hack (http://dietpi.com/phpbb/viewtopic.php?f=11&t=2832&p=10900#p10866) and selecting Kodi as autostart option.
Just bear in mind, updating DietPi will revert your changes (replaced with latest scripts), so you’ll need to re-add it back in.