On what basis is that belief held?
The earliest examples of proto-writing (essentially the etching of symbols which probably represented something, but not actual words in a sentence) only goes back to about 3,100 BC.
I understand well that if you believe Noah's Flood occurred 2,000 BC, these are pre-flood writings, which means we should also find them among the fossil remains of the dinosaurs, although we don't.
The problem with creation and the biblical timeline is that it doesn't correspond to anything we can observe. One account must be accepted on blind faith, and the other is based on observable and testable evidence.
There simply is no evidence that I'm aware of that the Books of Moses were based on earlier writings passed down from Adam. That people believe it anyway, I do not dispute.
The problem with the evos is that they try to insist that every creationist believes that the earth is a certain age old and that the Flood must have happened in a certain year when there's not one verse of Scripture that makes any statement about the age of the earth.