That sounds cool. I don't know if they'd want to federate with Lemmy and deal with such a huge amount of users - it seems you can't even view anything without logging in? And most are invite-only. So I'm guessing they like the small bookclub feel. Dunno how they operate with Mastodon or federate with each other.
Speaking of that, maybe in general instances could put up some gates that people from outside need to cross if they want to interact. Maybe just "write an introductory post" or something. Current it's either federate fully or defederate completely, and that just sucks.