This argument, of course, assumes that non-game analysis is completely objective. Anyhow, it's true that you can design your own biases into a wargame. If your sympathies are with the South, you might fudge, or overlook playtest results, or believe the wartime propaganda of either side. Unlike academic research, however, there is a way to test wargames for design bias. At the minimum the design can always be compared against the historical outcome.
Reality always trumps speculation.
Finally there is the objection of player strength distorting results. While you might like the history, it is still a game with game rules. Computer vs. computer play has pretty well eliminated this very real objection. In doing this a few times, you'll notice that you get smarter while the computer stays about the same. For example, in Age of Rifles the Antietam scenario begins in the dark.