I would say that the original argument has nothing to do with quantum theory at all, as this is about modal logic and its ontological implications, and perhaps I was in error to borrow Saul Kripke's example of "water is H2O" .

If I had used another example, for instance "Socrates is a man and all men are mortal", we could use this to make the same argument for unverifiable ontological truths. If we imagine a possible world in which Socrates disappeared and his body was never found, that would be a world in which his death is unverifiable. Nonetheless the ontological truth of his death would still stand in all possible worlds.

How fundamental these unverifiable ontological truths are is another question, which is why I suggested Plato was right to an extent--but to what extent, that is an unanswerable question, since not all ontological truths are verifiable.