That doesn't refute his underlying logic. It simply points out that there are parts of the world with less technology and less of an infrastructure. The demand for slaves nowadays -- even assuming it were legal -- is far less than it was 150 years ago. Hell, we're living in an age where people can't find work even if they want it. Automation, machination and computers have largely replaced the need for human labor save for a few areas -- sex comes to mind, although don't be surprised if that changes some day too.
You can preach all you want about cultural evolution, enlightenment, morals, God, etc. but people in masses are largely amoral creatures. If there's economic demand, the masses will utilize it, regardless of principles. The reason ~40% of people aren't slaves today like they were in ancient Athens is because there's a more efficient way to get buildings made.
Is a computer language with goto's totally Wirth-less?