It's almost definitely an error, but I don't think it significantly reduces the chances that Hersh's source is telling the truth. This feels like a detail that

It's almost definitely an error, but I don't think it significantly reduces the chances that Hersh's source is telling the truth. This feels like a detail that Hersh (incorrectly) added to the story, rather than something the source claimed. The charitable interpretation is that the aging Hersh confused Stoltenberg with his father, a Norwegian politician who was indeed involved with Vietnam. It's not a good look if Hersh is conflating different people from different generations, but it doesn't really reflect on the accuracy of his source one way or the other.