Say what?
Executive privilege derives from nothing but the implied power of a president to keep some matter confidential if the public interest requires it. That goes all the way back to George Washington. It was never really questioned until Nixon tried to use it to cover up Watergate-related crimes, at which time the courts ruled that executive privilege was not absolute. Then Clinton and Obama and the two Bush's tried to expand it and now nobody really knows where the power ends. Looks like we're about to find out.
For one thing, much of the information that could be covered under executive privilege involves interactions between the President and senior administration officials in roles that were created by Congress (cabinet members, for example) and/or subject to Senate confirmation. And Congress is responsible for funding every penny of the salaries of these people working in the executive branch anyway. In that sense the line between the executive and legislative branches is already somewhat blurred in terms of autonomy and oversight.