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.
Read that quote by Rob and the responses. IN the quote rob describes what he believes Generic Types are at face value... he literally takes it into a tirade about inheritance and hierarchies of classes... something that is not part of type theory at all.
Honestly, it feels like he didn't know about Algebraic Data types. I'm not the only one who thinks this as shown from the responses to his quotation.
One of the responses:
"Or perhaps Rob Pike just hasn't explored the relevant literature in enough depth. At one point he admitted that he didn't know that structural typing had already been invented previously! This isn't to criticize Rob, I find his talks fascinating, I think he's awesome, he's a friend of my boss, etc. But he's hardly the first hard-core hacker to be ignorant of the degree to which type theory has seen dramatic advances since the 1980s."