So its good because of Scala's ability to implement a programming language and not Scala's language itself being good on its own for general abstract sudo like code. Or does Scala write cleanly (like people say Python does) and is also elegantly able to expands it's own simple syntax?

It bugs me how languages like JS are clean but any custom domain-specific language made in them is ugly.

For the authors of this book, I imagine it was a combination of Scala having the features they wanted, plus it being relatively popular, easy to install, and having good tooling.

I've written a lot of Scala in the last 15 years or so, and I really like the language. It has features that most programmers are not familiar with, which can scare off some. My opinion is that if you understand the features it is a very elegant and simple language, particularly Scala 3.