Sorry about that, I'm tired of seeing people claim Nostr isn't decentralized always for the wrong reasons and never expand on their claims.
But in your case I think you really misunderstood things. The idea of Nostr isn't that messages are evenly spread across the network. I do not believe that is the correct approach at all. Although nothing prevents someone from trying and would be fun to see more p2p spreading of Nostr events I don't think that will scale, so I wouldn't focus on it.
Instead the approach Nostr takes is to let each person publish to one or more servers they choose. Decentralization happens by not requiring publishers to be present on a central location, but by allowing readers to go to whatever location necessary in order to fetch their content.
So even if everybody is using the same relay at one point, the network is still "decentralized" as long as clients are doing their job correctly and anyone can move out of that central relay anytime they want and start publishing to his own personal relay in his basement: all their followers will continue to get his updates automatically, now from the new relay.
It's explained visually here:
https://how-nostr-works.pages.dev/#/outbox
