Most Mormons -- and for all I know, all of them -- used to believe that American Indians were descended from the Lamanites. That is, the Lamanites were thought to be the primary if not the only ancestors of the American Indians. If one believes in the Book of Mormon, and in the absence of evidence to the contrary, that's a perfectly reasonable belief. Now we have solid evidence that such a belief is untenable.
So, do the Mormons say, "Obviously, our scriptures are false and our religion is disproven"? Of course not. They ask, "How can we reconcile the known facts to the scriptures we know to be true?"
That's what all believers do. It's what other Christians do; it's what Muslims do; it's what Jews do -- unless they just reject the facts completely.
Look at the genealogies of Jesus in Matthew and Luke. On the face of it, they cannot both be true. Many Christians "resolve" this difficulty by claiming that Luke's genealogy preserves the lineage of Mary, that is, by claiming that both accounts are still "true" because one of them means something entirely different from what it says. Others, more honest but no less determined, cooked up a web of Levirate marriages that contrive to explain how both lines can be what they claim to be -- the paternal lineage of Joseph. Only after many centuries did some of them finally throw up their hands and say, "Oh, well, the important thing is that Joseph was of the tribe of Judah and the house of David, and that's all the gospels are really trying to convey. The details aren't important." But at no point did any significant number of them look at the data and say, "Hey, look at this! Our religion is a bunch of crap!"
Likewise with the creation story -- or rather, stories. Attempts to make sense of them go back much farther than scientific theories about evolution. Now that we know there's no way they can possibly be factual, believers in the scriptures either deny the facts or come up with reasons for appreciating such obviously non-factual accounts; surprising numbers of them even claim that the creation stories were never meant to be considered factual at all.
Likewise with Islam. If all you have to go on is the Qur'an, it's not unreasonable to think that Maryam the mother of Jesus and Maryam the sister of Aaron are the same person. When Muslims learned that that was impossible, they didn't say, "The Qur'an must be false"; they said, "Obviously, the Qur'an doesn't mean she's literally the sister of Aaron. It means she's of the same people and lineage as Aaron."
I think this shows one of the problems with investing too much authority in scriptures, but it certainly doesn't mean the Mormons have a problem that's unique to them. Every group that believes in revelation and authoritative scriptures has this problem.
At least Katzpur has enough respect for the truth to accept the facts and try to understand her faith in the light of them. I talk to believers almost every day who are far less honest.