To convert the process into Mastodon terms, a community is like a dummy user, it’s not like a hashtag. Making a post to a community is like doing a mention tag to it, like @communityname@instancename, so the community’s home instance should always see all the posts to it. (This is the part that I noticed wasn’t working)

The community boosts every post to it, so every user who is subscribed/following the community can see the posts that it boosts, which means the instances hosting the subscribers/followers will receive copies of the posts, so federation is automatic there because of the boost.