But the power to admit a state to the Union is a power delegated to the United States, specifically the Congress. Once in the power to approve the splitting of stated or combining of states is also a power reserved to Congress. The power to approve or disapprove the changing of a states border by a fraction of an inch is also reserved to Congress. Implied in all of this is the power to approve a state leaving altogether.
The power to take any action that has a negative impact on the other states are also denied to the states. They cannot lay imports on the goods of other states, cannot enter into leagues against them cannot declare war against them or refuse to recognize their public acts. They cannot deny citizens of one state the rights their own citizens have. They cannot refuse to extradite fugitives. Given these kinds of actions are denied the states, then by implication unilateral secession and the negative impact it had on the remaining states is a power denied as well.
A whole lot of implying going on there vice what is actually written. We simply have a difference in interpretation.
Perhaps.