I have not read the book, but I respectfully disagree teaching a testing methodology should err towards over-testing. Your testing process should intelligently focus on the areas that most benefit from it, such as business domain logic, avoiding the pitfall of trying to cover e.g. your entire UI codebase.

Over testing can be as bad as not testing at all.

Well, I agree. But they're demonstrating testing. There's going to be a plethora of tests.