First of all, neither the Business Insider, Huffington Post, nor ThinkProgress are historical publications. The story from that Business Insider article is sourced from a website called Per Square Mile and is written by the creator of that site, Tim De Chant. He is also not a historian, nor is Per Square Mile a historical or peer-reviewed publisher. De Chant’s PhD is in Environmental Science, Policy and Management. However, as I was finding this out I was thinking this story did not have much hope, but it seems like I was too doubtful too quickly. De Chant sourced his information on Roman economic inequality from one of the world’s best peer-reviewed historical publishers for Roman history, the Journal of Roman Studies. The article was authored by Walter Scheidel and Steven J. Friesen, both PhDs in ancient history. The study they conducted is very comprehensive, and De Chant reports it accurately. Scheidel and Friesen conclude a Gini coefficient of 0.42 and 0.44 for the Roman Empire midway through the second century CE, while the Gini coefficient of the USA is higher at 0.45 in 2007. Therefore, I would say yes, the evidence suggests income inequality was less during the Roman Empire than it is today in the United States.

However, I would still advise people to practice caution when reading historical information from non-peer reviewed publications. It is easy for these things to become exaggerated. In the source article from the Journal of Roman Studies the authors urge reader caution and discuss the problems caused by a lack of “hard data” from the Roman period. Their numbers are provided within ranges, and even still might not be exact. Thus the stated Gini coefficient could easily go up or down a couple of points. De Chant does not transfer these cautions to his article, writing:

Schiedel and Friesen figure the elite orders and other wealthy made up about 1.5 percent of the 70 million inhabitants the empire claimed at its peak. Together, they controlled around 20 percent of the wealth.

Schiede and Friesen actually wrote, “some 1.5 per cent of households controlled 15 to 25 per cent of total income.” Thus De Chant provided a middle-of-the-line number that is not inaccurate, but without providing the truthful statistical range.

The Business Insider, that you linked too, goes a further step of exaggeration. The first paragraph ends with, the supposedly advanced and progressive United States of America is plagued by even worse income inequality. Their bold, not mine. You can be sure the Business Insider does not qualify their article with statistical cautions. Hence while not outright wrong, chasing up the sources of historical articles from places like the Business Insider is the best way to find out accurate historical information.