Did you watch the video he posted? That's not lag, they weren't even trying to kill them. They were too busy jumping on walls to even bother with them.

Natural occurring lag happens, you can damage each other, maybe not instantly, but hits will register eventually. Lagswitchers, on the other hand, can dish out damage and kill, while taking nothing at all - they become invincible. If it was natural lag, why are they trying to stand on each other's head in the first round, without some sort of upheld confidence that they won't die while doing this? It was certainly lagswitching.