The net worth of the richest people actually decreased. Actually, as with any generalization, that is true and false. Some of the richest people increased their wealth. Some decreased it. The fact that the net worth of the twenty richest dropped from $899B to $827B is a reflection of the drop in oil and commodity prices. It may or may not reflect a shift in wealth inequality because the net worth of the rest of the world wasn’t part of the study.
(Click on the chart for the link.)