While his points are most certainly valid, his technical terminology isn't. The AR-15 was full-auto-capable from its inception. Yes, It was a full-auto version that was adopted as the M16 service rifle. But just because most AR-15's are semi-auto does not make it incorrect to refer to a full-auto version as an AR-15, because it still is. Indeed, unless it meets the other criteria for the M16, it is nothing more than a full-auto AR-15.
I have a friend who routinely refers to his full-auto AR-15 as an M16, even though it's a 9mm carbine submachine gun. No, he happens to have a full-auto AR-15 in a 9mm submachine gun configuration. It is most certainly not an M16 in that dress. He makes it a point to correct himself in my presence, humorously.
All of this is aside from what he meant, and the obvious deception in the HBO interview. That said, he's an idiot if he expected fair editing.