The right to independence as outlined in the Declaration of Independence, makes all real property in the areas they inhabit, theirs, not the Federal governments.
If your theory were correct, the British would still be occupying forts in America. Your theory is clearly incorrect.
And no, they weren't "traitors" as the attempt to try Jefferson Davis for treason illustrates. Chief Justice Salmon P. Chase made it clear that "secession is not rebellion".
He also told Federal prosecutors that if they put Davis on trial, they would lose everything in court they had won on the battlefield.
If they can't declare Jefferson Davis a "traitor", I don't see how they could make any lesser Confederate into one.
South Carolina GAVE the feds Sumter decades before the Fire-eaters started agitating. It was federal property and was attacked by the “Confederate” forces under Beauregard. South Carolina gave up any legitimate claim to that property.
The Declaration had nothing to do with this and certainly did not justify acts of war against it (or any but ludicrous claims of ownership). British ownership of its forts predated the independence of the United States. Ownership was established by the Peace Treaty and Britain never claimed ownership thereafter even considering their slow surrender.