It is not such an uncommon thing to have apocryphal legends about family genealogy passed down generation to generation. One thing it certainly is not is Warren's fault. This is right up there with the birther stuff about Obama -- an ugly attack, sadly effective*, and we lose for even engaging in the discussion.
*during all the Obama years I don't think it ever truly sunk in for me just how real the "Kenyan Muslim B. Hussein Obama" stuff was. You hear about it, you see it lampooned almost everywhere, you chuckle about it and then you forget it because after all, look who won the election. Trump winning was a startling jerk to reality. And also, now you don't see it ridiculed in every decent forum because the f'ing President of the United States himself is the guy saying it. Which means not only does he command the prestige of the office with his megaphone, but the respectable outlets fall into the trap of covering the stupid s#!t he says like it's a topic that should be treated with gravitas.