As far as I know, WabiSabi doesn't have fidelity bonds. How does it defend against Sybil attacks?
There isn't a need for fidelity bonds with WabiSabi because no one is paying for anyone else's block space.
In a regular coinjoin, the coordinator must be relied upon not to orchestrate a Sybil attack.
I don't know what you mean by "a regular coinjoin". The coordinator is not "relied on not to orchestrate a Sybil attack", any non-coordinator entity can be a Sybil attacker:
Sybil attacks [Dou02] are an inherent threat to privacy in mixing scheme ...
I reviewed your research paper, and it doesn't explicitly address Sybil attack protection; it only briefly mentions it in section 7.2.2.
I answered these questions here:
https://bitcointalk.org/index.php?topic=5482818.msg63555300#msg63555300