It's federated, not decentralized. Which even Mastodon itself doesn't seem to realize or care, since they falsely advertise themselves as decentralized.

Decentralized means there is no central authority.

Federation just means there are many centralized authorities, that might or might not communicate with each other.

I really don't see what Mastodon is supposed to solve in the long run. The server has full control and can do whatever it wants. Just look at what happened Threads.net. Big company joins the Fediverse and instead of celebrating, everybody starts thinking about defederating them. This approach is doomed to fail if it ever gets popular.

Nostr looks like a much more promising approach, with proper cryptographic identities and signatures. Nobody owns you there. Servers are just dumb relays. If one steps out of line, you can just use another one.