All that content was stuck in each individual instance, so if I from nord.pub wrote a comment there, a user on lemmy.world would not see it.
We actually discovered right after the outage that Piefed instances appear to be able to see posts from other instances made to an offline or backlogged community. In particular, while viewing from piefed.social, I was able to see a lemmy.world user's posts to !climate@slrpnk.net before slrpnk.net had caught up on the backlog from lemmy.world.
What you describe is certainly true of Lemmy instances, though.