Case law does not supersede the Constitution, courts get it wrong all the time and there is nothing in the Constitution that says a state may not withdraw from the union.
no, it doesn't supersede the constitution. caselaw clarifies the constitution. caselaw says that it is unconstitutional for a state to secede, and until and unless that caselaw is overturned by another decision or an amendment, that's the law of the land.
Case law doesn't "clarify" anything. What it does is allow a bunch of hacks selected by politicians to subvert the Constitution the the benefit of those in power. It requires a supreme gullibility to believe that case law has any connection with objectivity or truth.
Lincoln's hand picked hacks decided secession wasn't constitutional. That decision is about as credible as the Alito Court's decision that Obamacare was a tax.