That stood out at me as unlikely as well. The hypothesis is essentially that wealth now captured by company founders, which in a previous era would been captured by their employers, explains the majority of the change in inequality. Essentially, that the main reason for increasing income inequality is that people who would've been salaried career engineers in the past are successful founders today. Some evidence in support of that hypothesis would be interesting. For my own part, I would be surprised if shifts in the share of income taken by W2 engineers vs. tech-company founders have even moved the needle on the overall U.S. economy's Gini index.
Their findings are that labor's share of overall income hasn't declined, but that wages/salaries have gotten much more unequal within the sector of employment-based income. Some of the factors they point to are: a greater polarization along skill lines; a decline in high-wage, skilled blue-collar labor; and a rise in the pay and number of high-end salaried jobs such as investment banking and C-level officers.
