> By that definition a void* pointer in C is a sum type.

No. That doesn't make any sense. void* is essentially equivalent to any in Go, which isn't sum types either.

You can construct sum types in C by combining structs, enums, and unions, but it is not an out of the box feature like in Go. Sum types are a first-class citizen in Go.

> Ergonomics matter.

Math doesn't care about ergonomics. You might care about ergonomics, but logically when talking about those ergonomics you'd call those ergonomics by name, not by some unrelated thing from type theory.