It's a natural logical assumption to make—Mandred is being trained to lead armies, and in Warhammer that means leading by the sword. He is also from the same order as Von Tarnus, so he has a institutional legacy, and is the owner of one of the best magical swords in the setting.
Like, if he was a canon character he probably would have the armour by default. Actually, he'd make a really cool lord—a heavy armoured melee combatant who can buff nearby units and shoot the occasional fireball for when he's not in combat. The Wizard Prince of Altdorf.
Of course, the nature of the quest means there's no guarantee that he'll hit the specific story beats needed to become that character, but it's probably easier for him to do so than become another type of character.