There's no monolithic "ruling class" that both major parties serve equally, and nor did the article claim anything like that. Instead, there is a huge patchwork of constantly shifting interest groups who are looking for suitable candidates to support, and candidates who are tweaking their platforms to appeal to as many groups as possible. Why would, for example, Fortune 500 companies spend a dime on elections if both parties served them the same.
The idea of a singular ruling class controlling the whole country is a lazy conspiracy theory that ignores the complexity of politics and governance.