tl;dr - Very specific way that lemmy.world broke means that there is a gap in activities that isn't going to federate

From a brief discussion on matrix with MrKaplan, it seems like the specific way in which lemmy broke for them (only really possible at their scale), the activities that happened during that period were marked as successfully sent even though they weren't. Going back and resending all activities from that point on could do really weird things and end up duplicating tons of posts/comments all across the fediverse, so they have opted to not do that. So, we will just have to live without those posts/comments in the wider fediverse unless you manually force federation for them.