I think Lewis missed the point.
I don't think the likes of Hawking and Musk are concerned about the fact that today's weapons can pick a target then guide themselves to that target without human intervention. I daresay they are aware of that (hell, Musk has been working with Nasa, so could well have military connections he wouldn't be allowed to broadcast. These weapons don't (AFAIK) generally launch without SOME human intervention, even if it's just a human telling them to launch.
I think what they are concerned about is giving that launch authority to a machine, so it can decide to attack people based upon it's own, arbitrary conditions. I don't know their full capabilities, but can today's missile systems go from being stood down (or however they normally are during peace time) to launch without any human intervention at all?