The same argument. If a coordinator doesn't have the reputation to attract enough users to put enough volume/liquidity into the coordinator then it can be sybil attacked by bad actors, and essentially make it function like their own honeypot. It's not a flaw in the coordinator, I'm merely saying that low liquidity = less effective CoinJoins.
Sure, more liquidity is absolutely an advantage when defending against a Sybil attacker. The minimum input count of the round is known ahead of time, so you can avoid choosing a coordinator that provides an inefficient/insecure service.