It is, but the content that is federated varies from instance to instance. Instances only federate communities that a user on that instance is subscribed to, so the all feed is a combination of posts from every community that an instance member is subscribed to. For a large instance like lemmy.world that’s basically every community, but for medium instances there are various small communities they don’t have and for a small instance, the all feed will reflect the interests of the founding members.