this is a nonsensical argument because the game is also disallowing tabletop OCs from settings that are completely removed from forgotten realms/eberron etc. essentially, if you do all of your own worldbuilding from scratch (included gods, regions, major NPCs, governments, alternate planes, whatever) and just use the dnd battle mechanics for your game, you still can't app your OC. i don't understand how you could argue that a dnd OC in that situation is a "fandom" OC, and most of the dnd groups I know do that for their campaigns.