Hello,
This is a guess on my part but could the problem they are having be something to do with the Pi4 having two HDMI outputs? I ask this only because on my Pi3 I did a complete fresh install for Bullseye including installing Kodi and I get the same error but Kodi does load and displays its user interface on the TV.
I am having my own problem though of not being able to navigate using my TV controller anymore since the fresh install of Bullseye and Kodi. This is probably down to something I am doing wrong like a setting that needs changing but at the moment I am lost on how to fix this.
Thank you for DietPi I have been using it for a few years now on a Pi3 and Pi4 it is my goto.
ISquishWorms