You are correct not all those who meet criteria for NPD are psychopaths. And not all psychopaths are narcissists. Psychopathy is not in the DSM or the englsih equivalent diagnostic system (ICD, ICD-10 being the latest version). Since Hare devised his psychopathy checklist - psychopathy has become of more interest. You can even go and do a "course" to hearn how to use the Hare Psychopathy Checklist. It's pretty expensive. There are other's in the field who disagree with this "diagnosis". In DSM there is "Antisocial Personality Disorder" - which has some of the features of psychopathy.
The article you site, is not from a medical journal.