My only annotation here is that I think MDC was added in the early 2000s, after HMAC. My understanding is it's not so much that we hadn't figured out real authe

My only annotation here is that I think MDC was added in the early 2000s, after HMAC. My understanding is it's not so much that we hadn't figured out real authenticators, it's that the people designing PGP didn't believe in them or believed them unnecessary for some reason. You can see the echoes of this silliness in the description of MDCs in the RFC today, because of course RFCs are for pontification and not for spec description. (Grep RFC4880 for "NON-NORMATIVE EXPLANATION" if you're interested.)