Antietam was a Union Victory

It is commonplace to refer to the Battle of Antietam as a "bloody stalemate" or some such -- Wikipedia has "draw" and "tactically inconclusive." Civilwar.org has "While the Battle of Antietam is considered a draw from a military point of view, Abraham Lincoln and the Union claimed victory" and later uses Union "victory" in sneer quotes.

Now it is true that thanks to McClellan's cautious bumbling, what could have been a stupendous Union victory became something less.

But it seems to me what if your enemy invades your country, and you send your army to stop him, and there is a battle, and he turns tail and marches rapidly back the way he came, that counts as a victory. Or am I missing something?