I don't know if defensive aggression is the right term. Or maybe it is for you but defense is reactive. It doesn't really belong in Schutzhund and dogs generally don't do well if they are defensive . You might be able to make a personal protection dog with that though.
Perhaps you mean active aggression. Active or social aggression is the core of the protective instinct. It's how the dog starts the fight with a helper. Not where the helper starts it and then determines everything the dog will do. It should be the other way around, where the dog dictates what the helper does...
I think many would agree that the jargon used to discuss aggression, drives, etc. in dogs is not known for its congruence. Also, I believe drives/behaviors overlap and the constructs we use to describe a dog's behavior have limitations and we can only speculate based on experience, what a dog is feeling, which would reflect his drive states. When I was referring to my dog, I can see why the term defensive aggression is not fully accurate and there could be an element of active aggression, since the dog is not being threatened and shows no avoidance, outs quickly and is forward. Just to muddy the water, Winkler uses the term active defensive aggression. He says it is a reactive form of aggression, because all defensive aggression is reactive, but is strong, powerful and stems from confidence in the dog. But he kind of contradicts himself when he says defensive aggression is always triggered by worry and then says with active defensive aggression, "offense is the best defense." He also sees social aggression as a very specific trait where a dog with true social aggression will bite anyone outside of his pack because they are outside the pack and defensive aggression is not part of social aggression. That doesn't describe my dog. He is generally social. For a while, I thought he might end up being the type of dog you could take out of his crate and walk away with, but with maturity, that is not the case at all. I do think the dog is starting the fight with the helper because, as I said, the decoy is not agitating the dog in any way, and if I out him, he stares the decoy in the eyes and is eager to reengage. What I see on the bicep bite is a combination of frustration, anger and some stress, which is fading, because the dog is young and has not fully learned what to do during the fight. His grips are very full, hard and don't shift, which tells me there is confidence behind them and I see improvement with each session, so the dog is working through any uncertainty he might be experiencing. Also, a properly socialized dog tends to have some natural inhibition to bite a human because people are seen as sort of an adoptive relative. Once the taboo of biting is overcome via training, that uncertainty goes away and becomes a behavior the dog looks forward to voluntarily do, rather than feeling he has to do it out of self preservation. The self defense is more of a purer form of defensive aggression, and that is not what I am seeing.