The U.S. Secretary of the Treasury speaks for the President; it's a fundamental dynamic of organizations. Otherwise, effectively Yellen wouldn't be Treasury Secretary - Yellen would be powerless and meaningless - and would resign or be fired. Only Trump seemed to ignore this and undermine the people under him. Also, I expect that the Treasury Secretary has great legal authority to make binding decisions for the U.S. government; remember that the American people decided the cabinet members would be separately confirmed by Congress (i.e., the Senate), per the Constitution.
Similarly, if the CFO of Apple makes an agreement, the counter-party assumes they speak for CEO Tim Cook. Otherwise, why talk to this person?
> Treaties in the US require a super-majority (two thirds) vote in the Senate.
Most international agreements are not treaties. The people of the U.S. delegate the power to conduct foreign affairs almost exclusively to the President, again in the Constitution. Only certain actions, such as treaties, require Congressional approval.