The United States is currently linked by all sorts of agreements and treaties to countries around the world and organizations like the UN. Does that render our separate expressions and acts of sovereignty meaningless as well?
Not one of these states made a separate treaty with a foreign nation, the real test of a nation.
You are incorrect about that. IIRC there were a couple of agreements made between one or two colonies and european nations during the revolutionary war (I pinged 4CJ on this in case he remembers which ones and the dates). Some of the states also made treaties with each other during the revolutionary war, the first being Georgia, South Carolina, and the Cherokee Nation over borders and indian relations in 1777. And of course there is the 1783 Treaty of Paris ending the war, which was signed between "His Britannic Majesty" and the "free sovereign and independent states" of "New Hampshire, Massachusetts Bay, Rhode Island and Providence Plantations, Connecticut, New York, New Jersey, Pennsylvania, Maryland, Virginia, North Carolina, South Carolina and Georgia"
And of course that was seven years after establishing the United States of America. And of course thsat was two years after unanimously ratifying the "perpetual Union." And of course that treaty was negotiated by the representatives of the United States of America (rather than the individual states).