There's no requirement for state parties to sign up to the treaty. In effect there's no difference between each signatory implementing national laws independent

There's no requirement for state parties to sign up to the treaty. In effect there's no difference between each signatory implementing national laws independently from the treaty or implementing them because they're required to under the treaty. It's voluntary to join up. If the holy see, which hasn't ratified the treaty, wants to launch nuclear weapons from the Vatican then they're entirely welcome to do so. I'm sure there'll be objections from other avenues but this treaty is not one of them.