Sion Ult Is Canceled by Syndra Ball [Gameplay]

We weren't able to test it on our own because none of my friends own Syndra, but we were playing an ARAM today, and Syndra was able to use her Scatter the Weak to knockback Sion during his ult. The ult was canceled afterwards. It happened both times that she used it on him, so it seemed pretty repeatable. I'm pretty sure he should be immune to knockbacks.