there actually is a way to "backfill" content, being the outbox. (although it was not meant to be used this way, from what i can see from reading what little documentation is out there)
lemmy uses it to federate about 10 or so (i think it was?) of the latest posts, and most microblog implementations use to federate the "pinned" posts of an account
activitypub has quite a lot of quirks, both spec-wise and implementation wise, and there are many reasons including an apparent near civil war inside the working group which resulted in this messy state it's ended up in
honestly the fact that this protocol works at all is a miracle