Nostr uses bitcoin taproot as identity and schnorr signatures. It's literally the first sentence of the nostr spec:

"Each user has a keypair. Signatures, public key, and encodings are done according to the Schnorr signatures standard for the curve secp256k1."

Bitcoin identity can be used with relays. It can be used with nostr front ends (npubs), it can be used in financial transactions (taproot). And so on. It can be used powerfully in multiple contexts.

You could indeed call it bitcoin identity, but we are talking on nostr, so calling it nostr identity makes sense. It all boils down to the same thing.