Interfaces in Go are structurally typed but they're still types. A variable of an interface type has two components: a pointer to its dynamic type information, including virtual method table, and a pointer to its value. When you consider that any compiled Go program has a finite set of known types concretely implementing each of its interfaces, they essentially become discriminated unions, albeit without Rust's compact inline representation (unless the dynamic type is itself a thin pointer).

> it’s anything in the world that someone might decide to write an isSumType function for.

No. Notice the lowercase tag name. It is impossible for anyone else to add an arbitrary type to the closed set.

Unless your argument is that sum types fundamentally cannot exist? Obviously given a more traditional syntax like,

   type SumType tagged {
      A | B
   }

...one can come along and add C just the same. I guess that is true in some natural properties of the universe way. It is a poor take in context, however.