From the context he seems to just be talking about code coverage as a metric, not TDD (though maybe he goes into more detail in the book?).

TDD claims to lead to higher coverage but I guess they'd claim to help you avoid missing logic paths too. Since you start with tests and then write code so that you've always got high coverage. If you've not written a test for it then you can assume that it doesn't work. The alternative is to write a bunch of code, then add just enough tests to keep a coverage metric happy.