Yep, but the binary running in the enclave has access to both the attestation document (including PCRs) directly from Nitro as well as a mechanism to fetch IAM

Yep, but the binary running in the enclave has access to both the attestation document (including PCRs) directly from Nitro as well as a mechanism to fetch IAM policies and verify that they are from the genuine IAM server (verify the TLS CA). Making sure the IAM policy hasn't been tampered is just a case of adding logic to the enclave app to make sure that the IAM policy is configured correctly for that particular enclave (compare PCRs, make sure there's no wildcards, etc.)