The book is more about explaining how things work and showing how they're implemented in a simple way. With this premise I think Java is a fair choice because almost everyone knows it. If you're writing a book where you're showing every single line of code of the implementation not having to explain the programming language used is a huge benefit.

Regarding type checking, I believe it wouldn't be too hard to add it to the language itself, but it would increase the page count of an already long book.

All things considered I think the book does a good job of explaining the concepts in a very accessible way without handwaving any detail.