If you are indeed the coordinator then you have got an edge, you could simply reject all other users in the round I am joining and the attack will work, but again, doing that would be obvious because other users who have valid inputs will be rejected for no good reason and that would indicate the coordinator is attempting the attack.
Couldn't the coordinator just use own inputs and 'prioritize' them over real user inputs whenever they need to do a sybil attack? How would that be obvious to other users? It could easily be that those are not coordinator inputs but that it's real user demand that's simply higher than usual for a brief period of time, no?
I don't think users get something like a timestamped proof that they submitted inputs to a CoinJoin at a certain point in time (could be used to show that they entered the CoinJoin before the coordinator started to attack), right?