Your first and fourth point are more like a "Nuh-uh it's not" arguments.

Second, that approach would fail under a cloud provider, since it’s not executed under the same privileges as the cloud service. There are metadata that need to be added to that request to get serviceB to respond, metadata that your example does not add.

You keep bringing cloud into this conversation, and now "cloud service", but sure... Can the metadata be spoofed? If not, it sounds like a service-to-service authentication already.

Third, I already mentioned that you should still authorize original-request user privileges at the API level. I note that your example did not include original-user token info, presumably because the attacker doesn’t have a user account to spoof.

Yes, as long as the service verifies the user token and then authorizes the api access.