ActivityPub addressing is really somewhat complex and pretty much any server is going to abstract it with things like at-mentions, sometimes making different assumptions. When I reply to lemmy posts from my microblog.pub server, I sometimes need to make sure to include both the user (@elena) and community (@fediverse) if they happen to be on different domains. Otherwise my server's addressing won't send it to the right place. And I doubt other lemmy instances will see this post unless I tag them too.