Trying to summarize here:

The open letter says: "If we allow autonomous weapons, a global arms race will make them much cheaper and much more easily available to terrorists, dictators etc. We want to prevent this, so we propose to outlaw autonomous weapons."

The author of the article argues, that the technology gets developed either way and will be cheaply available, and then continues to say, that autonomous weapons would reduce casualties in war.

I suspect that most people agree, that (if used ethically) autonomous weapons reduce casualties. The actual question is, how much (more) damage can someone without qualms about ethics do with autonomous weapons, and can we implement policies to minimize the availability of autonomous weapons to people we don't want to have them.

I think the main problem with this whole discussion was already mentioned elsewhere: Robotics and AI experts aren't experts on politics, and don't know what the actual effects of an autonomous weapon ban would be.