I don't know how the secure enclave works in detail but if there is a private key inside it that it uses for attestation / signing, presumably it could also hav

I don't know how the secure enclave works in detail but if there is a private key inside it that it uses for attestation / signing, presumably it could also have a certificate signed by an internal Apple provisioning CA infrastructure which Apple can verify on their end. Importantly, this matters even for those older devices that were created without secure enclaves. iMessage still used this PKI architecture back before every new mac/iphone/ipad had a SE.