That statement is a truism, which is not entirely true. Not counting military ammunition during war time the lowly .22 LR probably has killed and injured more people than any other round. Why? Because it is universally available, cheap and common-place. At one time this round was preferred even by professional hit men because weapons chambered for it were cheap, easy to ditch and so numerous that they were virtually untraceable. The round is also quite deadly. When fired out of the short barrel of a pistol or revolver it normally has just enough 'power' to penetrate the torso then it will tend to ricochet all over the place inflicting serious and even life-ending injuries. Same thing when fired out of a rifle at further range. In other words, it seldom makes for a clean through-n-through wound.
Does this make it the deadliest round in the world? Consider the following: The best one-shot stopper yet is the .357 Magnum followed closely by the .45 acp. In all-out war the military calibers have probably accounted for far more deaths than the .22 LR even though it still is certainly one of the top deadliest rounds in terms of loss of life.