Look up NewTypes. The article's dance is to avoid having extra fields that are completely unnecessary here. They want some kind of nominal email type, that is

Look up NewTypes. The article's dance is to avoid having extra fields that are completely unnecessary here. They want some kind of nominal email type, that is actually a string, so can be used in places where a string is needed, but when a method requires an "email" you can't use any string. It's a pretty common pattern in functional programming and in many other languages nowadays