Their logic is an answer (e.g. Noah's flood happened) in search of questions. They presuppose the flood occurred. Therefore everything is evidence of that. Its also the fallacy of affirming the consequent.

If you try to refute it logically, you'll never get anywhere because it's not based on logic. It's a multi-layered self-referencing logical fallacy. All they do is move goalposts.

As far as how this doesn't make sense. It's the fallacy of Occam's razor: basically given two answers the simplest one is the right one. But the world is not simple at all. And in scientific circles, there is no empirical evidence supporting Occam's razor in Earth Science. Quite often, the correct answer is complex because these are open, complex systems.

So what's more likely? All the geologic layers were caused by a single event thousands of years ago? Or, these layers are the result of millenniums upon millenniums of processes that cumulatively resulted in our planets geology?