Author here: a few people have raised this point. Perhaps I should have made a distinction between "federated within the same data center" and "federated across the internet." They are two entirely different problems.
Right, but since Twitter manages all of Twitter's servers (and the dedicated low-latency network via which they're presumably all connected) they can satisfy both of these constraints without sacrificing the third (immediacy). I'm speculating here, but I think that's why it takes a few seconds to post a tweet: it's propagating across the internal federation so that all the nodes have it before it "goes live."