Their history was oral. Doesn't make it any the worse as a history - nor any less divinely inspired for many years of being passed down word-of-mouth before it was first written.
Heck - They had not yet invented the “zero” - so how do you expect them to keep track of decimal places and powers of ten?
But their entire SEQUENCE of events describes what we only now know. And THAT is the miracle of Genesis.
In any case everything we know about modern history was penned by either eyewitnesses, or people who have researched the events and penned their own accounts. The Bible is no different, except that God has guided the recording process, to keep it error free. Just because we don't have the reference materials that Moses used, does not mean they did not exist!
Who wrote Shakespears plays? We don't have the original manuscripts, so perhaps they were handed down by oral tradition as well.