Cleave isn't instantaneous, Cleave is literally a pointblank Dismantle that only works when Sukuna is able to amplify the level of Dismantle based on the opponents CE levels(Basically, it's intended to always be able to damage an opponent severely regardless of their defenses)... The problem with that is that if the opponent is stronger than Sukuna, Cleave is effectively meaningless since he's only ramping it up based on his OWN CE levels... and Super Aoko technically has infinite Magical Energy, meaning Cleave just means effortless free shot from Aoko to literally cleave Sukuna in half with a kick or an "afterburner enhanced" punch. He doesn't even need that, despite what dumbasses think, Kiritsugu DOES know how to use Magecraft, even typical instant Hypnosis better than Waver's. He just tends not to use it because "it's easier to fuck with Mages using apparently weaker shit to open them up to an Origin Round or blow them up". And again, they forget Kiritsugu has a month of...