Well, I'm very fluent in Typescript, and often find JS code extremely difficult to understand. The types are a great aid to understanding code. It also seems to be the case that a lot of JS code does not use ES6 polyfills, unlike Typescript code, so it tends to be less clean.

Edit: Upon rereading I see you are talking about the book, not actual projects, so my reply is not all that applicable. Though I'd actually say that Typescript is much more understandable than JS to someone unfamiliar with both.