The events in the game very often hold to the historic timeline.
I don't think it would be possible to reference IF there were a battle during the previous turn nor its outcome.
I personally would not assess the actual battle of Antietam as having had a high NM gain. The only thing that defined it as a US victory was that Lee retreated from the field. But losses were heavier on the Union side, while having more than twice as many men on hand, plus Lee managed--or McClellan allowed Lee--to leave the battlefield anaccausted. The only fact about this battle that allowed Lincoln to present it as a victory was that it was the first major battle in the East in which the Union army not only remained on the field after the battle, but did not retreat back to the Washington defenses. It was more the situation of finally having not getting routed after having taken heavy losses. I think most did not look closely at the huge force disparity at the start of the battle, and even less the disparity after the battle. It was really a pretty bad example of a "victory"