Because like the author says, there is no substitute for competence. If you can't write good code, then your assumptions about the future design will be wrong and/or bad, and your tests will be as poorly written. In other words, TDD (anecdotally) doesn't improve your code but only introduces extra levels of complexity. Which in the hands of an incompetent developer becomes an even bigger problem than if there was no TDD.
If you can't write good code, then you will not write good code.
But, there is another common case where unit tests (written before or after) also come in tremendously handy: if you can't write good mistake-free code 100% of the time.
Which describes all programmers who have ever existed.
