Fediverse defederation forces a degree of soft group consensus on moderation: it's possible to say "if you're talking to X, I don't want to talk to you".
How? If server A blocks server B, and neither server blocks server C, server C can still interact with both servers A and B. Server A could of course choose to also block server C for not blocking server B, but this would have to be done manually (you can't necessarily tell if server C blocks server B, since blocklists are often not public and not all interactions are public either) and I don't see how it's forced.