Doctorow seems to contradict himself, answering a question about legal, not technical, barriers by describing what end up technical barriers.
But I don't know how this ideal would translate into implementation anyway. ActivityPub is built on walled gardens at the instance level, which may be better or worse than one centralized walled garden, but it still leaves users at the mercy of the folks running the instances.
It's a huge fault in the core design of ActivityPub.