> they do have comparable uses, so it makes sense to wish for similar syntax to reduce cognitive overhead

No this makes no sense at all. Set theoretic types don't use special constructors, there is no way to make them look remotely similar. He is clearly not aware that these are different kinds of unions.

> They do.

Nope, precisely because they are less common. You yourself also clearly also didn't understand the difference between "untagged unions" in C and set theoretic type systems.