You also need to remember khadgar + we the hero +tons of other NPCs are in open dialogue with both factions and could have easily solved the misunderstanding. Also this is what emissaries and messengers are for. On top of this, both factions are literally present in dalaran in shouting distance, can easily explain other sides story.. Out of all the excuses , oh alliance had no way of knowing what really happened to the horde so first thing they did in the middle of a fucking burning legion invasion is to attack horde is the LOLest excuse. Adding to this, the news of vol'jins death surely reached alliance by now which should indicate why they might have fucking retreated