Most of what Khadgar did was in the opening scenario. We didn't see much of him outside of the legendary ring quests. And he didn't ultimately thwart Gul'dan in WOD. Not by himself anyway. Sure we storm Hellfire Citadel and slay Archimonde, but Gul'dan ultimately escapes, and Khadgar fails to stop Gul'dan in the Tomb of Sargeras plot, and he's far from the only character involved in the Nighthold as well where Gul'dan fainally IS thwarted. Gul'dan gets a leg up over Khadgar more than once during WOD and leading into Legion.