A system is decentralized when no single party can unilaterally break, censor, or control it. Nostr is decentralized at the identity and the transport layers: your keys are yours, your events live on many independent relays, and any single relay or client vendor going away leaves everything else intact.
Decentralization is a spectrum, not a binary. Nostr's feed ranking, moderation, and discovery all still depend on clients making choices, and clients can become centralized gatekeepers themselves. The protocol only gives you the raw material; using it in a way that preserves the property is on the humans who build and use the clients.