Darius ult still deals damage while he's stunned midst animation

So I was playing against the new Darius and this happened. As you can see he gets stuck in my (Viktor) W and gets stunned while ulting but I still get blown up even though Darius doesn't finish his animation. Seeing as Darius' ult isn't an AoE channel type of ult unlike say Kennen, Fiddle and Swain, which don't get stopped by stuns, I don't see why Darius can still get his ultimate damage off while he's stunned. Where's the room for counterplay?