If the United States is a Corporation there should be evidence in the Congressional record. All I found is this record from 1871 that creates a body corporate for municipal purposes for the District of Columbia. This is the District of Columbia Organic Act of 1871 and happened during President Grants Administration. A govenor is appointed for a 4 year term who is now called the mayor of DC. I'm not saying there is no evidence but a quick search turned up nothing.
Did Derek Johnson say the entire US is a corporation or just DC? His demeanor and presentation made concentration on his words and message a bit challenging. I got through until about :47. Will try to listen to rest later.