The fact that a treaty or in this case was legally binding upon the citizens of the state in no way indicates that the state was agreeing to forever bind itself and surrender its sovereignty. Any law passed by the state government that does not conflict with the state constitution is binding upon the citizens.
That’s why it was called a “perpetual union” and later a “more perfect union” - because it was meant to be enduring (AKA permanent).
And yes - the states (individually) DID surrender a portion of their sovereignty when they became members of the union.
Where do you get the "treaty" malarkey? It wasn't a treaty. The states ratified the Constitution. They didn't form an alliance. They agreed to be bound by the Constitution, regardless of any misconceptions they may have had about what they could and could not do.