Sum types and multiple return values are not mutually exclusive, though several languages today use tuples to emulate multiple return types (see Scala, Rust for examples).

I am not a language designer, but I have become interested in languages in the past couple of years. Sum types require some sort of generics implementation, which Go does not have. I think the design choices the authors made regarding the language have made adding generics that much harder, that now they are struggling to find the "Go way" of fitting them into the language.