In that regard the conclusion (regarding wealth inequality) really isn't that surprising. If the ROI of having money is really higher than the ROI of earning money (i.e. by wages) then the conclusion over time is as simple as the solution to a binary star system containing a black hole... eventually through the iterative process of the economy any given dollar is likely to fall into the basket of the rich guy, and it is much harder to escape that basket than the basket of the poor guy who needs to spend simply to live.
The article doesn't seem to conclude that capitalism is flawed though (at least to the extent that Marxism is called for), but that our actual capitalist system needs corrections made before revolutionary corrects get imposed by force.