User blog comment:Hamengeri/Kit Concept: Melee anti-caster utilizing Mantra/@comment-7709681-20140814162147/@comment-7709681-20140814220011

Disarm only disables autoattacks (And comes from a game in which being Blinded doesn't guarantees missing autoattacks), Amumu's ult is Entangle (The only case of it in the game actually, and our Entangle is the same as in other MOBAs), which disables movement and autoattacks but not abilties.