Is it though? It kind of feels like you broke up with someone but are still getting constant updates about who they're currently fucking the minute you left.
Every time it's mentioned it's only about sending e-mail to different providers, but the analogy doesn't cover for example browsing your e-mail inbox and seeing communication from multiple sources. Just covering how one individual can send messages to multiple individuals via a common protocol is only half the picture.
Honestly it's more like the old mailing lists (Majordomo days), where individuals would subscribe to a list, but that list might also subscribe to other lists themselves, and then you throw a web interface in front of it.
I know you're using the e-mail analogy to represent how SMTP and ActivtyPub are a common protocol, but I've seen the same analogy mentioned several times and I think it only serves to muddy the waters, because it's incomplete.
This isn't Facebook grandpa, you need to show your work.