The truth is, we all tell lies. Occasionally, one of the participants in Dr. De Paulo's study would insist that he or she could be entirely truthful for three or four weeks. None of them ever succeeded. (SUMMIT 2, pp. 16, Joan Saslow, Allen Ascher)
What does the underlined part mean? Does it mean that there are some people who do not lie? Or, it may mean there aren't any people who succeed?