In fact, Executive Privilege is itself a myth unfounded in the language or original understanding of the Constitution. The Constitution was not supposed to give presidents power to withhold information from Congress.
When a president invokes Executive Privilege, he is saying that despite a congressional request for information, or even despite a congressional subpoena, he is not going to let Congress learn what it wants to know. He, in other words, knows better than Congress what matters Congress should investigate. Lew Rockwell.com