> It can be difficult to communicate that each nostr app is a interface onto a single common database, rather than a separate silo. But this isn't the case, is it? Each relay is a separate silo. Nostr's website says: "Clients fetch data from relays of their choice and publish data to relays of their choice. Relays don't talk to one another, only directly to users." Or have I misunderstood?

Relays can get posts from eachother, how else would the propagation of messages even work otherwise ? You post to x and y relays. z and a relays can read and repost your post like anyother user client. There is nothing preventing another relay from reading a JSON file like any other client.

"can" isn't the same as "do".