Actually the constitution is talking about rebellion, not secession, there is a difference.
Chief Justice Salmon P. Chase..
“If you bring these leaders to trial, it will condemn the North, for by the Constitution, secession is not rebellion.”
The only difference I see is "rebellion" is what you call a secession when you want to de-legitimize it. It is a sophistry game to pretend forcing people to abide by your rule is moral.