> 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.