I personally think that he takes into account some factors flight sims cannot model at the moment. Control forces and harmony being the most important i guess.
In AH you can fly endless aerobatics without getting tired, but in a real fight that won't be the case.
Turning circle in reality might in many cases much more the result of the pilots ability to apply back pressure on the stick than of the wingload.
Erich Hartmann himself referred to turnfights as being a competition between the physical fitness of the involved pilots. He knew several comrades that trained their fitness to the point where they just would fight so long with an opponent till he lost due to fatigue.
This is probably it.
Since I have no flight experience, much less aerial combat experience whatsoever, I can only rephrase some of the info I've read, but it seems pretty clear that pulling black out turns at speeds again and again would hardly be anything a real life pilot would want to do.
In AH we can go from +5G to -3G in a matter of seconds. We do 'stick stirring' where the vector of the plane is changed violently in a split second, and then still instantly pull out of it to go into another high G turn yet.
Most pilots would hardly have the time to manage the trinkets, buttons, levers inside the cockpit. Some go as far as to say that changing the throttle setting during combat wasn't a common thing to happen.
It's not difficult to imagine that in WW2 planes, which a pilot had to fight the stick forces at high G loads, the pilot would rarely, if ever, let go of the stick with one hand and start fiddling with throttle levers or push switches or buttons.
In most cases the throttle would be set upon a certain combat setting, and then the pilot would hold the stick with both hands and maneuver at that state.
Based upon past discussions, especially the one related to flaps, it also seems pretty clear that even the "combat flaps", were not really used all that much in combat. Only a handful of extremely experienced pilots would ever use it - in great many cases the flaps were never used in combat at all. People openly admit that they can't find any real any evidence of extensive flap usage for the P-38s. Others with correspondence with P-51 pilots mention that no P-51 pilot ever recalls having using flaps during combat.
...
So basically, my picture of a real life "dogfight" between a P-51 and Fw190D-9 would consist of two pilots with typically throttle set firmly to combat power.
Both pilots would hold the stick firmly with two hands, and struggle to pull turns at speeds. Neither side would go as far as to approach grey out.
Using turn-tightening techniques such as chopping throttle, dumping E, using flaps and etc.. is probably a grossly uncommon thing to happen.
Both planes, as a result, would probably go around in simular turn circles most of the times.
At least, that's my take on it.
Just for the sake of discussion, if HT somehow adds an experimental system of fatigue... the fights between planes would show a lot less diversified results than it is now - at least, in the turn fighting department.
