"the fediverse model is confusing people" is a far cry from "it's a single app on a vertical platform", as you say in your original post.

Meanwhile, I only see content from the Nostr relays I'm using, and not others. Surely, this is *much* more confusing - I have one username that works everywhere on Nostr, but if people aren't subscribed to the same relays as I am, they won't see my messages. This is "the future"? 

 That's not how nostr works. People can see content from you though they are not subscribed to te same relays.