No, it's still federated. But my account of alice@foo.com follows bob@bar.com Bob registers his new Twitter handle so I, Alice, can find it on the central serv

No, it's still federated. But my account of alice@foo.com follows bob@bar.com Bob registers his new Twitter handle so I, Alice, can find it on the central service of Twitter. A mapping of users to users can be across any service, including centralized to centralized (ie Twitter to Facebook) or decentalized to decentralized (IE Diaspora to Mastadon). It is just saying this string registered somewhere else as that string.