If I remember correctly from the interview (I think it is here), one conclusion was that TDD doesn't have a clear benefit when you add it to a project. On the other hand, in a survey, TDD projects are more likely to succeed because it is a habit common to good developers. I hope I am capturing the subtlety there. Essentially, TDD is not a silver bullet, but rather a good habit shared by many good developers. That was enough to convince me of the merits.