Probably because he has i-frames for a few moments after tp'ing or you had a network issue, but him having i-frames make more sense in this case.
tbh i think it was the network but why would there be i-frames that late in the animation
or it could have been weird hitboxes during the animation
I don't know, maybe riot didn't anticipate anyone being stupid enough to place their tp out in the open and actually taking it and haven't realized it has a weird hitbox or something