Nonsense. They agreed to abide by the provisions of the Constitution when they were admitted to the Union. They were admitted by a vote of Congress.
Nonsense. There's no agreement to "abide by the provisions of the Constitution" anywhere. It does state that the 'Ratification of the Conventions of nine States, shall be sufficient for the Establishment of this Constitution between the States so ratifying the Same.' So, implicitly, the convention of a state can withdraw it's ratification. Forget implicit, the Constitution EXPLICITLY states, 'Full Faith and Credit shall be given in each State to the public Acts, Records, and judicial Proceedings of every other State.'
They were admitted by a vote of Congress.
The originial states were not. And SCOTUS has repeatedly held that the latter additions were admitted as equals of the original states.