Any military game will have controversy when generals have ratings and ranks and their historical position can be a bit vague. For the most part, this game allows promotions for leaders who had both positional (ie - appointed) leadership as well as functional leadership positions. McCulloch is a bit of an odd duck. He was a two star leader in the Mexican-American War, but in the Civil War he was only a one star leader even though he held a post of great responsibility and even led a wing of the Confederate Army.
As all CSA leaders were one star leaders initially, had McCulloch survived Wilson's Creek he may, indeed, have been promoted, but historically he was not.
When I play a game I am not looking to see if the designers 'got it right.' Instead, as when reading a book of history, I am looking for the designers interpretation of battles and leaders.