tldr: chapo.chat isn't ready yet.
Don't worry, you'll get to be called libs by the users from lemmygrad.ml Soon™.
[chapo.chat is] running a heavily modified version of lemmy forked months ago, for not just the front end, but the back end as well. Once they’re up to date with the most recent lemmy versions that have federation working, then we can try federation, on the back end at least. Their back end devs are great, but their front end devs contribute nothing upstream, and will have to re-create all the federation-related parts of the lemmy-ui code for their own fork.
EDIT: See @Beatnik's comment
Three at the moment (of any significant size), the other two mentioned in the post that are much smaller than us, and us.
what are the other two? I know of communism.lemmy.ml, but what's the other one?