The way posts are shared between instances is by user subscription. For example, if there's a community on Lemmy and a Kbin user subscribes to it, Kbin will then receive new posts from that Lemmy instance for that community.
So if no one on an instance is subscribed to that community, new posts won't flow to that instance. And then if you do subscribe to it, the instance will only automatically receive new posts. Federation will not back-fill older content.