This is incorrect. They can't make "agreements" with foreign countries but that's because of the Constitution they all agreed to, not because they aren't sovereign.
> supreme power especially over a body politic
> freedom from external control
States aren't sovereign. The state legislatures don't even control their Senators anymore. The fiction of "collection of independent states" is just that: fiction. If nothing else, the American Civil War put an end to that. The Senate is just an unjust system which causes California to have to remit 33% of taxes to other states, while those are state's Senators vote to strip the rights Californians wish to have.