Secession is not a constitutional right; it is the natural right of all men.
It is, however, not settled in a courtroom, but by force of arms. That doesn’t make it less of a right.
Let me expand on that a bit. As the Declaration makes clear, the power of government derives from the consent of the governed.
If it loses that consent, it follows that it loses the legitimate power of governance. That government will not legally recognize this is unsurprising; it has always been so.
So, right there in the Declaration, secession is basically endorsed by implication. That does not mean that it is constitutionally prescribed, however. It is, again, not a legal right, but a natural right.
So, basically, I think Justice Scalia is right; he’s just omitting part of the answer.