AFAIK that’s just the default way that federation works on the fediverse. Subscribing to communities is not retroactive, it will pull over a handful of posts sometimes but basically you only start copying the content fully from the moment that the first user of your instance subscribed. Same way with Lemmy instances.
It does make sense because it would rapidly increase the resources required to host a server if it was mirroring that much more content with every subscribe.
I’m not sure if there is a way to add this functionality, but it would be really beneficial to have a toggle that allows a server admin to enable retroactive subscription in certain cases. It might require some technical improvements but it seems like a key feature.
They do have a history of removing threads and posts that get too popular though. I remember several incidents where highly upvoted comments and posts about Lemmy got removed for seemingly no reason.
It’s probably impossible for them to entirely prevent discussion about Lemmy so they instead astroturf and try to manipulate the discussion to portray the platform in a bad light. It seems to be an extremely effective tactic, unfortunately for us.
Reddit’s obfuscation of upvotes and downvotes is problematic, and makes it trivial for them to manipulate any discussion if they feel like it. Not to mention their ability to just nuke anything they don’t like with no repercussions.