I agree that Nostr is a public bulletin board. But it's not a timestamping service. Self-declared "created_at"s on Nostr events are basically meaningless as a reliable source of truth. Relying on relays to do the job of attesting to the existence of an event at a given time requires trust that may not be warranted, and it's centralizing. Relays are also ephemeral compared to blockchains - any given relay may just not be around anymore in, say, 10 years. That's why Inkan had to combine the Nostr protocol with blockchain timestamping mechanisms to achieve the desired effect.