Except it is, there is a reason statistic samples do not need to have thousands upon thousands of numbers to be accurate. As long as you have a sample that is correct enough (which this is). If the top 200 are not accurate enough for you, then your samples will never be good enough unless you take the whole population. It is highly unlikely that the majority of the OKW players are just all strait up better than all the allies players, the same could be said of when the Soviet win loss ratios were out of control
The sample quality is fine here, you could take a larger sample size and the results would not change very much (because this sample was chosen adequately).
It seems you want the sample to be absolutely perfect, which is absolutely absurd
It doesn't matter. There is no way to prove it is otherwise neither.
For this to be true you have to assume there is an equal number of games for each of the sides + all of these games were against opponents of equal skill. We all know that's not possible hence this is all one big assumption and can't be treated as a research. You bend the data to suit your ideology.
All this indicates is how it's the win ratio among 200 players for all of the factions, but that's not enough to draw any conclusion from this as there are too many variables.