Even more importantly, these farmers were, for the most part, slaves. The article is describing a society where for every one "middle class person" just like us, there were 20 slaves working full time to support him. It is then comparing the living standard of that one person surviving off essentially 20 slaves to the living standard of the average person today in a society with no slaves.

Of course, as long as this is emphasized then the article really does do a good job of making it clear how much things have improved for everyone on Earth in the last 500 years.