I don't think it being unfalsifiable is a problem. I think this is more of a definition than a derivation. Morality is a fuzzy concept that we have intuitions about, and we like to formalize these sorts of things into definitions. This can't be disproven any more than the definition of a triangle can be disproven.
What needs to be done instead is show the definition to be incoherent or that it doesn't match our intuition.