one shot kills through smoke is actually relatively easy if you get your timings right - especially at lower levels where the guy chucking the smoke nade to cover a narrow door opening follows through as soon as the smoke appears - done a few of those myself (and been on the receiving end)
that said the video you're talking about (if it's the same one I'm thinking on) did look a bit sus, but then he could have just gotten lucky. or he could have been posting the video in the off chance someone from the forums got it on overwatch and would give him the benefit of the doubt![]()
IIRC, the video that I watched, the smoke had been allowed to cover the entrance/doorway and the suspect waited for a second or 2 before headshotting the guy at the other end, I think he was using that new OP/broken weapon too, the revolver that has the trigger delay?
Like I said, it could just be luck or/and pure skill, although it happened way too often to be considered luck imo and I think he said that video was all from 1 round too? Generally when you see good 4-5 minute FPS compilations, it is taken from 5+ rounds of footage.