There are WAAAAY to many factors that aren’t spelled out to us for us to make a determination of any correct/incorrect play of strategy in a fantasy world. Was it lunacy for Anduin to go in there? Who knows. Did they have the building already surrounded? Did they believe the Horde leader to be cornered? Did they think that Sylvanas, being a dark entity, would be weakest to Anduin, a light entity? Was Anduin showing strength to his army and their many sacrafices by showing he wasn’t hiding in the back lines? Were they certain that Sylvanas wouldn’t use the plague since it was a “war crime” (whatever that means in WoW) to use it?
I mean, dude… I could go on. The fact is that this is a fantasy story and the scene that we DON’T see if an ever shifting landscape that is (usually) always presumed to support the seen stage.