If that's the case, he's doing it wrong. It's like the pot calling the kettle black. He essentially argues that types are constraining on other languages based

If that's the case, he's doing it wrong. It's like the pot calling the kettle black. He essentially argues that types are constraining on other languages based on types, where it is actually in Go that types CONSTRAIN your use of things. Because obviously different things CAN be held in a container, and a container can hold different things -- but Go doesn't let you do that (except for the officially "blessed" containers). It is Go that's concerned more about "what something is", in letting you work with it, than, say, Haskell.