I think the article cracks open the long-held assumption that presidents have very little to do with national economic performance. It doesn't prove or disprove anything, but it does show a stark pattern that has been true in the USA for 14 presidents, 22 presidential 4-year terms, over a time span of 88 years, nearly a century. The economy has grown significantly faster under Democratic presidents than Republican ones – strong evidence that a president may very well have an effect over the economy.