Pyke un stun-able during ult

So apparently you cant stun Pyke at all during his ult animation which does true damage. im velkoz, i hit a perfect E on Pyke as he is jumping up to do his ult (Pyke is at the VERY beginning of his ult animation, just jumping up), but my E doesnt stun Pyke or even do dmg to him. Pyke ult was also doing close to 800 true dmg, and Pyke had built Cinderhulk jg item, warmongs, spirit visage, deadmans plate, and a black cleaver, and his ult is doing close to 800 true dmg. I get that Pyke gets AD instead of Hp from HP items, but still over 700 true dmg, and you cant stun Pyke at the very beginning of his ult animation. GG Rito