Parent commenter said "basically" have 100% test coverage, and I'm not sure what is meant by that. From the text[0]:
> Any program feature without an automated test simply doesn't exist. Programmers write unit tests so that their confidence in the operation of the program can be part of the program itself. Customers write functional tests so that their confidence in the operation of the program can become part of the program, too. The result is a program that becomes more and more confident over time -- it becomes more capable of accepting change, not less.
> You don't have to write a test for every single method you write, only production methods that could possibly break. Sometimes you just want to find out if something is possible. You go explore for half an hour. Yes, it is possible. Now you throw away your code and start over with tests.
[Emphasis mine.]
I can see how this is interpreted both ways, so I'll leave it to you. For what it's worth, I found the book to be an easier read than most books on coding practices.
[0] Extreme Programming Explained by Kent Beck, Chapter 10 under "Testing"