It is.
> I was expecting union types (e.g., https://www.typescriptlang.org/docs/handbook/unions-and-inte.... or https://dotty.epfl.ch/docs/reference/new-types/union-types.h.... )
Union types are basically an anonymous form of sum types. In the same way tuples and structs / records are both forms of product types.
Statically typed languages which support sum types generally only support the named version (because it tends to be more useful and powerful).
> I guess enums are a type of sum type, but seems this is more specifically about enums
Depends on the language, C's enums are not types at all, Java's or C++'s are product types. Technically enums (or more generally tagged unions, which is what Rust's enums are) is a superset of sum types as you can have multiple variants of the same type, but that's not leveraged here.