Just because the enemy doesn't realize they are sailing into a decisive battle that can only be decisive against their goals doesn't mean the battle is any less decisive.
Well, yes and no. The Japanese were idiots - they never asked themselves how a battle could decisively knock the US out of the war. Midway ended the Japanese capacity for major offensives at sea, but it didn't have even the potential for knocking the US out. (As I'm sure you already know.)