Manpower has everything to do with history the moment they attached it to a real life historical event where it can be refuted. If they would have made up something (Hitlers birthday! recruits are flocking to the military!) we couldnt see peep, but when they attached it to a real life event, where we know it didnt occur, they opened themself up to be evaluated.
Let me ask you something if you edit the anschluss to give Germany 100 manpower, and then linked it to a follow up event that provided another 400 manpower (Baby Boomers population growth congrats Germany!). How would the game be effected negatively? germany would still get their 500 manpower. That is all im saying. No other issues need to be brought into play here. The end result is the same, its just more palatable historically. We know they had events that arent historical because it adds flavor and or helps the game be "balanced" so we tend to live with them without much discussion, why not simply do the same here, especially if its so easy to do. Why all the discussions about the game, its mechanics etc etc, when another simply automatic event would have handled it properly.