1620 was after the advent of the printing press and mass production of paper and the spread of reading and writing generally. By then we recorded everything.

We don't know as much about who conquered whom in pre-Colombian America, other than standout examples like the Incans, Mayans, and Aztecs. Oral histories fade rather quickly especially when decimated by war, famine, or disease. But even when conquered and absorbed into a society, how quickly would the descendents forget if properly integrated? A few generations is all it would take. We speak English because there was a society known as the Angles that I know almost nothing about. Are there any pure blood Angles still around? Who knows? They were conquered by the Saxons and no one can today tell you the difference. I'd reckon that the Anglo or Saxon distinction went away rather quickly.