If "message passing" is like "mail comes to your house", a "shared heap" system works differently
In a "shared heap" system, all the mail gets dumped at the post office, and in the most naive version, you go over there and read through every single piece of mail to see which one is relevant to you
There is no "directed delivery" in a "shared heap" system, which means you are stuck with two things: either a "god's eye view" (Bluesky) or "even lossier about replies than ActivityPub" (Secure Scuttlebutt/Nostr)