They are using Byzantine Fault Tolerant Quorums. That is just a class of majority election algorithms that is tolerant to the presence of evil participants. But

They are using Byzantine Fault Tolerant Quorums. That is just a class of majority election algorithms that is tolerant to the presence of evil participants. But in short, it is just the majority of servers allowed to vote, so either proof of stake or a cartel. What they neglected to specify is the difference there: "who is allowed to vote?"