That’s how I understood the explanation from https://lemmy.world/comment/205763 as well.

We can still do that. However, the “true” version of those communities are the ones on beehaw, so our posts will not be shared to other instances via ActivityPub.

If you alternatively create your post on Rammy and Beeaw and Lemmy.world users reply, it’s Beehaw who suffers the most in that situation because they don’t see any content from lemmy.world users while all other instances see everything. You can read a specific example in that link under third instance communities.