An instance is a community, period. This is like trying to argue that subforums on a forum is not a single community. They may be smaller blocks within a larger community, but they still form a larger community as a whole and adhere to a generalized ruleset.

You're the one that keeps comparing it to email, so I recommend you stop. Simple as that. Ctrl-F this thread and every response that has to do with email starts with you. I've explicitly compared it to IRC and forums.

Your last point is incorrect. Beehaw is a reddit alternative. They defederated from other instances because other instances had free registeration, which was resulting in users from that instance trolling and doing low quality posts in their community (according to them). They can do this because the instance is the community.