Horse feathers!
After the collapse of British authority in 1775 it became necessary to form new state governments and by the end of 1777 ten new state constitutions had been created. Connecticut and Rhode Island kept their colonial charters but removed all references to British sovereignty. Massachusetts waited until 1780 to adopt it's new Constitution. All of this prior to adoption to the Articles of Confederation!
The were already free and independent states prior to their becoming party to any union whatever!
And did 13 colonies issue 13 declarations of independence? Or did they meet in a single congress and approve and issue their declaration from that single body?
After the collapse of British authority in 1775 it became necessary to form new state governments and by the end of 1777 ten new state constitutions had been created.
And well before that they had been meeting in Congress - first the First Continental Congress in 1774 and then the Second Continental Congress that began meeting in May 1775.