The Bluesky approach to the "shared heap" is that *everything* goes into the big, centralized shared heap. Bluesky takes a "god's eye" view: it knows everything, and so knows what all your replies are, and can give you perfect search.
Secure Scuttlebutt / Nostr... well long story. Lossier, I'll say
You can imagine the physical world version of "message passing" already because you already live in this world. Messages come to your house or apartment building or whatever
For Bluesky's "shared heap" architecture, you'd have to build a whole addition to your house for everyone's mail