Ok, you got me there. I concede this arguement to you.
Ironically, Jefferson lost a separation of powers argument during the treason trial of Aaron Burr. He resisted releasing a letter that Burr claimed would exonerate him, saying it was inappropriate for the court to compel the president to do anything. After the court said he was not above the law, he "voluntarily" submitted the letter - to avoid setting a precedent.