The point is that such moment by moment destruction is spontaneous (ākasmika) and is the uncaused real nature of things, because it cannot be an effect of any cause. The effect of such a cause, i.e., the absence of the entity, would have to be a type of non-being (abhāva), and non-being is unreal.
A key underlying principle of the vināśitvānumāna is that negative facts, such as absences, are not part of the ultimate furniture of the world, but are just fictional conceptual constructions, as they are devoid of causal powers. Equally, a fiction lacking causal powers is not the effect of something else. While it is obviously impossible to deny that hammers smash pots, the absence (abhāva) of the pot, i.e., the non-existent pot, is not an effect, just as other non-existent things (abhāva), like horns of rabbits, are not effects of anything either. Hammers and the like are thus not actually causes of the pot's absence but of it turning into potsherds. That idea is perhaps defensible, in that arguably the mere absence of something—a purely negative fact—might be less real and less efficacious than the presence of other things. Nonetheless, the rest of the argument looks to consist in a number of non-sequiturs going from that difference in efficacy and reality between absences and presences to the idea that perishing is somehow the real nature of things, that it must be intrinsic to them, and that therefore things must perish spontaneously moment after moment. Let's grant the Buddhist view that the perishing of x is the real property of changing into a new thing, and not just x becoming absent. If it is accepted that hammer blows do change pots into potsherds, then why couldn't someone skeptical about the Buddhist's arguments just take that as the model of how things perish when they do? It does not follow from that model of perishing that a pot could not endure for quite a while.
Is it a non sequitur?
If there are no absences then is nothing excluded from the causal context of an effect? If so, then couldn't we just conclude that causation is beyond conceptualization, so that any cause (the hammer) of e.g. destruction (the broken pot) is also not the cause, and the effect of destruction must be "the real nature of things", so invariant?
Is that a good reconstruction of the argument, or have I misunderstood?