Well, if I recall this right, this statement was based on assumption where Moses had access to written records describing those early days of humanity. Records were written by people who had experienced most of it. My memory might trick me but as far as I remember it was clearly accepted that Moses was collecting the data from other sources concerning times before him. Of course those records Moses had access to are gone for a long time ago, as God had no reason to store them as he stored Bible to future generations.
Using fuzzy logic you get from here for JW acceptable statement that oldest part of Bible predates other scripts, not because it was actually written prior to them, but because scripts older than other religious books were used as reference material when Bible was written. Ofcourse it was impossible to write religious books before mankind appeared on earth and what comes to cave paintings, they are ofcourse dated wrong as conditions changed dramatically in flood
CP