Kayn Ult Bug
Kayn's ult is not making him untargetable.
What happened in the vid has happened multiple times.
I have heard from other people saying this has happened as well.
Some said it was possibly the DOT from DD and Red Smite. I thought that at first but it also happened when i was bot lane against Thresh and Cait that had no DOTs.
casting W on you while your Leeching someone else/or via him