Honestly this is really compelling evidence that Pike doesn't know much about type theory. That isn't terribly surprising, and the other early collaborators on the language that I know of also came from more of a systems background. I think its entirely likely that Go's crippled type system is partly an accident, and not entirely a design choice. It would be helpful if - with the benefit of hindsight - they would admit it, rather than invent post-hoc justifications for the way things are.