But I think you know where I am going with this, right?
Nothing was done explicitly through legislation. Nothing was done to amend the Constitution.
Calling this issue settled from a constitutional perspective is like calling Dredd Scott v. Sanders settled constitutionally speaking.
At best, it is wishful thinking.
Are you familiar with the legal concept of “stare decisis?” When a Supreme Court ruling has stood as precedent for 148 years it is very unlikely to be overturned. Dred Scott was case law for only 9 years until the Civil Rights Act of 1866 (and then the 14th Amendment in 1868) rendered it moot.
Certainly Congress has the power to pass legislation that would permit secession of a state but I see that as highly unlikely. In the current Senate, such legislation would be filibustered into oblivion.