I believe he was speaking metaphorically. Obviously, the US played a huge role in the ETO, and France and the rest of Europe would probably be speaking German or Russian if it weren't for the US. But again, that's my opinion...
The D.520 was a good aircraft for its time - albeit a rather short life in terms of quality of performance. The reason was simple - France was taken out of the fight very early in the conflict.
What if the Arsenal VG.33 had achieved operational status? It was easily on par with the 109 and Spitfire. The problem wasn't with the quality of the aircraft they were building and designing, the problem was with the manufacture and production of those aircraft.
The French were notoriously unwilling to believe that the Germans would be capable of occupying France, especially in 1940 when France had a large, numerically superior military force. While most of their equipment was comparable to that of the Axis, the numbers, lack of training and doctrine, and overall poor morale of her armed services was what allowed Germany to run roughshod over France in a matter of weeks.
Is the D.520 filling a hole in the existing planeset? No, not from a performance standpoint. From a historical view, yes, it should be there....but so should the Bloch MB.152, the Moraine Saulnier Ms.406, Breguet 690 series, Potez 630 series, etc. etc. What about the Curtiss Hawk 75s flown by the Vichy? Far more numerous than the D.520 or Ms.406, more than 400 being sent before occupation.
We can ask for, and receive, every single plane to achieve operational status in WW2, and someone will still want something obscure. Adding new aircraft isn't the answer to our problems - improving the quality of gameplay with entirely new features is what's going to make the difference, artillery, SAR, new targets, terrains, etc . Eventually, the structure of AH2 won't be able to support all the the things we want to do, and we'll be forced to move to AH3, and this argument will most likely start all over again with "We need a Spitfire!"