With stuff like this, from a theological stand point it's not which story is oldest but which one is correct.
I will continue with the realization I am a Christian theologian so will speak from that perspective.
The fact is, the Egyptian sun and Middle Eastern moon based religions were indeed older than Judaism and that is why you see the book of Genesis refer to the sun and moon as "great lights" so that there was no onus on Jews to rever them as anything more than created beings. Other religions around them however were talking about them being gods.
Same with the flood. Where many other older religions have the same story, the Bible tries to correct the account.
This being said, for game purposes do whatever you want. Maybe there's another religion, that the GM made up, that has the true story. Could make an interesting adventure after all.
Historical gaming, like fantasy gaming, can go any way you please. I've published the book as inspiration for such things.