Interesting. The “political disunity” the article is talking about means that when a significant portion of the rich people in the country starts to fear being made poor, then the country is liable to break up. This is essentially what happened during the American Revolution: the rich Americans felt they could no longer count on being given a fair shake by the British Monarchy so they broke away and created their own country.
History supports two narratives of rising inequality leading to social disintegration and political instability: one is inequality between the top classes and everyone else, and the the other is rising inequality within the top classes. When the pie starts shrinking and there aren't enough slices to satisfy the rising expectations of the top class, the elites splinter in profound political disunity.