OP is simply incorrect.

I’m coding a Lemmy alternative right now and have been testing this functionality out extensively. Deletes of posts and comments certainly federate, I’ve seen the AP traffic to make it happen. Also, the docs: https://join-lemmy.org/docs/contributors/05-federation.html#delete-post-or-comment

I haven’t tested what happens when the ‘delete account’ button is clicked… Mastodon solves this by sending a ‘delete this user’ Activity to every fediverse instance so there’s nothing about ActivityPub that makes removing an account and all it’s posts in one go impossible.