When you go to /c/books on your server, you don’t see an agglomeration of all /c/books on all servers of the fediverse. You only see that server’s /c/books, if it even has one.
What prevents from visiting /c/books@anotherserver?
Genuinely asking, because this is one of the core concepts of Lemmy and federation
Importing posts and comments could cause a security risk if someone would to abuse that function.
Even Mastodon doesn’t support it
Mastodon currently does not support importing posts or media due to technical limitations, but your archive can be viewed by any software that understands how to parse Activity Streams 2.0 documents.