

Since email is no less secure than snail mail
I would disagree with that. The attack surface on snail mail is much, much smaller (only whoever can get in physical contact with my mail) and any attack scales incredibly badly. It is also often hard to read snail mail without making it obvious that it has been tampered with (i.e. opening the envelope).
Meanwhile the attack surface of email is huge (basically the entire internet), any attack can scale wildly and it is impossible to tell if anyone else read an email.
By and large, physical stuff is much more secure than digital stuff, just less convenient.
Just want to counter this: Privacy is in fact a part of ActivityPub. Stuff is only meant to be public if it is sent to the Public collection, otherwise it should only be delivered to the intended recipients, much like email. This is part of the core protocol, not any extension.