My supposition is that he was referring to 18 USC 2384, Seditious Conspiracy, which reads as follows--and I've bolded the portion that could arguably apply to this factual situation:
If two or more persons in any State or Territory, or in any place subject to the jurisdiction of the United States, conspire to overthrow, put down, or to destroy by force the Government of the United States, or to levy war against them, or to oppose by force the authority thereof, or by force to prevent, hinder, or delay the execution of any law of the United States, or by force to seize, take, or possess any property of the United States contrary to the authority thereof, they shall each be fined under this title or imprisoned not more than twenty years, or both.
I think diGenova's argument is that Obama and the Intel Community were part of a conspiracy to "overthrow" the Government of the United States.
Obviously that argument has to address the meaning of "overthrow." My counter argument would be that the context appears to require that any seditious conspiracy must be a conspiracy to use physical force or--at a minimum--a credible threat to use physical force in some real sense. I believe the history of the use of our sedition laws bears out my interpretation.
Nevertheless, it seems to me that Obama was treading an exceedingly fine line. It's not much of a step from urging the violation of key intel officials' oaths of office by withholding national security information from the Commander in Chief regarding the purported actions of a hostile foreign power to the possibility of advocating mutiny in the armed forces. And that definitely would be a seditious conspiracy.