Is two shotting the enemy really skill expression?

Depends on how massive of a radius the misleading hitbox of your 3-second cc "skillshot" truly is

Just as much as is

As always context is king. The question isn't "what did they do" (two shot the target), it's how they did it.

If I have linear collide on contact skillshots and the enemy team is full of Yasuos and Cho'Gaths and whatnot defending their hypercarry, and I'm still able to pick off their hyper carry in two hits, definitely skill expression. If I have Righteous Glory and Gargoyle's stoneplate and pre-nerf Cho'Gath, that's not really skill expression at all.

If you got extremely fed, meaning you are now able to one shot people, also meaning that you were able to outplay people to get fed.