“Non-falsifiability” in reference to cryptographic assumptions has a precise technical meaning: “an assumption is falsifiable if [and only if] it can be modeled as an interactive game between an adversary and an efficient challenger that can efficiently decide if the adversary won the game”. Most standard-model assumptions used in cryptography are falsifiable in that sense. OTOH, models such as the random oracle, ideal cipher, generic group, or algebraic group models, which are not equivalent to assuming any falsifiable assumption, are also in common use.
@zawy, I think you’re misunderstanding this term: it has very little to do with incompleteness. Nor is falsifiability by itself particularly useful as a standard for cryptographic assumptions to be held to; for example, a trivially false assumption is falsifiable.