I can't confirm for certain without going back through all the interviews, but what I remember her saying about which missing DE was the one who "left [him] forever" was that 'a lot of fan sites had it right,' rather than giving the actual name. I wavered for a bit, but finally it made more sense to me to have Karkaroff be the "cowardly one" (he denied Voldemort to save his own skin, then fled when he returned), while Severus - who was outed as a spy and had presumably SWITCHED loyalties - would be the one who had "left [him] forever." Just my take.