What is important to note is that they don't address the Sybil attack on a technological level:
> Deploying BigchainDB in a federation with a high barrier of entry based on trust and reputation discourages the participants from performing an attack of the clones. The DNS system, for example, is living proof of an Internet-scale distributed federation. Appendix C describes how the DNS has successfully run a decentralized Internetscale database for decades.
As far as I understand with spending only little time on this matter that's the main tradeoff they took compared to fully decentralized databases such as the Bitcoin's public ledger and which allow them to scale the performance.