No actually its because msgs “boosted” by the community will only be sent to your inbox post-follow, if that makes sense. For the same reason when you search for and sub to a community on one Lemmy instance to a comm. on another, it only gets the last few posts; but your instance will get all the ones that come after.

Essentially following a lemmy community is like a magazine subscription; you don’t get all the previous ones retroactively, but you get all the ones that come after you subscribe/follow.