You nailed it, it only pulls posts from communities that someone on your instance subbed to. It doesn't even pull retroactively; your instance only starts pulling posts created after the first subscriber on the instance subbed.
I'm more concerned regarding media, because just like Mastodon, the pics themselves are copied from other instances onto yours. I hope it will be enough to just find -mtime -delete
once in a while
Frankly I'm not sure what it does haha. What are the "best communities"? Which communities? Also what does it actually do, subscribes all users on your instance to those "best communities"?