Once you actually read it then you'll find out that it does:
> What do our findings say about democracy in America? They certainly constitute troubling news for advocates of “populistic” democracy, who want governments to respond primarily or exclusively to the policy preferences of their citizens. In the United States, our findings indicate, the majority does not rule—at least not in the causal sense of actually determining policy outcomes. When a majority of citizens disagrees with economic elites or with organized interests, they generally lose. Moreover, because of the strong status quo bias built into the U.S. political system, even when fairly large majorities of Americans favor policy change, they generally do not get it.
The study literally concludes that regardless of what party is in power, it's the interests of the oligarchs that take precedence. Whenever there is a difference of the opinion between the masses and the oligarchs then the oligarchs typically win. That's whose interests the parties represent.
And, if you think that's what a democracy is then what else is there to tell you.
You claimed that both parties represent the exact same interests, and I don't agree with that at all and nor does the article support it.
If we use campaign contributions as a proxy for interests, then internet companies like Google and Facebook and their employees support Democrats much more than they support Republicans, and it's vice versa in the aerospace and defence industry.