It should be the responsibility of the poster to ensure their message gets to all their followers. Not the responsibility of the viewer to find all the posts from people they follow.
ActivityPub doesn't have this problem because it uses an inbox model. This makes sense because posters should be motivated to ensure their content reaches their followers.
In Nostr when a user makes a post, we should enumerate their followers and find the best relay for each by NIP-05 or NIP-65. Then deliver the event to those relays. This is essentially what ActivityPub does.