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.)
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.)