The author uses the term "union", but unfortunately they picked the one wrong term against a long list of correct alternatives: sum type, tagged union, discriminated union, coproduct, disjoint union, variant, algebraic data type, ...

"Union" does not imply disjointness (which is the source of many problems), whereas all those other terms do.

You’re probably thinking of C/C++ unions, and they do indeed have quite tricky semantics and are difficult to use correctly. What the article and the other commentators are talking about is more properly called a “sum type”, like Rust’s enums, for instance. Those are different things from C unions. In languages with first class support for sum types, they are used everywhere, it’s an incredibly useful concept.

Not at all. Untagged unions, sure, but no decent modern language has first-class support for untagged unions. First-class tagged unions are a real treat to use.