Well, you are right. This article sort of sucks in explaining anything and it is also already outdated. Understandably your summary of how it could work is wrong and the article is indeed to blame.

My suggestion would be to skip it and learn about nostr from other sources. I'm on Nostr since almost the beginning and it's been very exciting to watch. For reference my android client app (Amethyst) is currently directly connected to 390 relays (using the new "outbox model") and it works well, no slow down, no battery drain.