Walt is right in line with the father of the Constitution, James Madison. Madison said quite plainly that unilateral secession was not constitutional.
Your response fails to address my statement - that Walt's conclusion about secession does not logically follow from his premise. Madison's opinion may certainly weigh in on another aspect of this debate, as may that of Jefferson, whose writings permit the possibility of secession. But neither of those authorities are the issue at hand. Therefore your citing of Madison is a non-response.