Correct, the coordinator fee does nothing to prevent against a malicious coordinator since they would just paying themselves.

However, even third parties are still able to sybil attack Whirlpool for no marginal cost even with the coordinator fee. The coordinator fee just slightly increases their up front cost to attack, while the continuous cost of attack remains zero.

Wasabi coinjoins fix this free sybil attack by requiring attackers to pay for their own mining fees instead of forcing victims to pay for the mining fees of their attackers.