I don’t see how it’s relevant. Bitcoin uses CPU power to establish a global consensus or “canonical truth” of Bitcoin’s history. Nostr and spam aren’t really relevant, other than PoW could maybe be used to protect against spammers flooding a relay by having non-paying or non-invite or non-WoT users provide a small PoW in order to broadcast to the relay specifically.

Otherwise the idea of competing processing power doesn’t apply unless we are all trying to find a single global state and agree on which is the “real” one.