That is simply not true.
The first time secession was presented to the Court, its ruling snuffed out the concept of legal unilateral state secession.
From the majority opinion in The Prize Cases
"....By the Constitution, Congress alone has the power to declare a national or foreign war. It cannot declare war against a State, or any number of States, by virtue of any clause in the Constitution. The Constitution confers on the President the whole Executive power. He is bound to take care that the laws be faithfully executed. He is Commander-in-chief of the Army and Navy of the United States, and of the militia of the several States when called into the actual service of the United States. He has no power to initiate or declare a war either against a foreign nation or a domestic State. But, by the Acts of Congress of February 28th, 1795, [The Militia Act] and 3d of March, 1807, he is authorized to call out the militia and use the military and naval forces of the United States in case of invasion by foreign nations and to suppress insurrection against the government of a State or of the United States."
Legal secession has no basis in United States law.
Walt