But so what? Why think the Noah story originated from a pagan source?

There is no reason to think the Noah story predates the other stories, say skeptics; we do not know which originated first. Indeed, it is probable that all the stories had some common origin, due to their striking plot similarities. This is not positive evidence that the Noah story (the exact date of which is unknown) was derivative, but it is evidence that the story underwent a oral retelling process that resulted in distortion, in at least some if not all cases. So why think the Noah version is the original?