I think NDT thinks that theist, agnostic and atheist are on the same spectrum, where as they are two different spectra. You can be Gnostic or Agnostic, and Theist or Atheist.
This chart can help to understand:
So, by being agnostic you state that it cannot be known if god does or does not exist, and by being gnostic you state that it can be known.
So, I think what he's trying to say is that he isn't a gnostic atheist but that he's an agnostic atheist. The fact that he's not an activist atheist shouldn't have any effect on his belief on the existence of god, and on him being an atheist. He just doesn't want to be labeled as one because of the burden the word carries.