I suspect that most people agree, that (if used ethically) autonomous weapons reduce casualties.

What does "if used ethically" mean?

This is a bit like the debate around tasers. Taser seem like a good idea because they allow policeman to use less force. In reality in nearly every case where a policeman wanted to use a real gun in the past they still use a real gun. The do additional shots with the tasers.

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.

The US is already using it's drones in Pakistan in a way that violates many passages of international law, like shooting at people who rescue wounded people. That's not in line with ethical use. They use the weapons whenever they expect that to produce a military advantage.

Robotics and AI experts aren't experts on politics, and don't know what the actual effects of an autonomous weapon ban would be.

Elon Musk does politics in the sense that he has experience in lobbying for laws getting passed. He likely has people with deeper knowledge on staff.

On the other hand I don't see that the author of the article has political experience.