Just to clarify, best in faction for ITC has never been based on sub-factions without their own codex like World Eaters (or Ravenguard or Tallarn or whatever). The app lets you pick them but they get resolved for tracking ranking under the parent faction (in this case World Eaters counts has counted Chaos Space Marine/Hereitic Astartes and still counts for that now). Been that way for ages and they have a spreadsheet that shows what counted for what: https://docs.google.com/spreadsheets/d/16p0r1w3IHR2xGrIGpWREMhs9Amnqzbti6TGG8-xiSyE/edit#gid=0
With the new change to how you determine ITC faction, they added god specific ITC faction. However you determine faction also by specificity (with Chaos being less speciific then Khorne and Khorne being less specific then Chaos Demon or CSM
). So if your army was World Eaters, despite being all Khorne, CSM
is actually more specific. Hope that helps!