At runtime every value of any of these types are tagged in some some way. The struct based ones with an explicit tag member that is not visible at the language

At runtime every value of any of these types are tagged in some some way. The struct based ones with an explicit tag member that is not visible at the language level. All the rest by way of the object's runtime type (v-pointer). Which means the fact that some look at a language level like a traditional closed sum type, and others look more like a union type is pretty much just that, looks. Even the "ad hoc unions" are basically functioning as sum types here, just an ad hoc sum type whose type constructors are other types, and abusing the fact that classes or boxed structs all have a vptr that can be used as the discriminator. This all compiles down to code that pattern matches on either the explicit tag member, or on the runtime type, and after the pattern match you basically have a normal type to work with. Hence why they are lumping it all together.