Here’s their site. The data sources are all referenced there. I’d like to see you debunk them.
http://politicalcalculations.blogspot.com/2011/10/real-story-behind-rising-us-income.html
If the number of people with very high incomes shrinks as their income increases, and those with low incomes become more numerous as income drops, in aggregate, the Gini index will not change.