And, they should not unless they live in the Americas and cross pollinated. It doesn't matter if it's the Bible that says so or the Book of Mormon. The debate is why doesn't the indigenous people's DNA match with the Jews in Israel? You are trying to disprove the Book of Mormon using the DNA excuse. But, then you dismiss Lehi altogether. Your reasoning isn't even circular. It's disjointed.
So, Lehi is necessary to be a real person to make your claim. Lehi was of the tribe of Joseph. Joseph's tribe was carried away in 721 BC and never known about since. I'm sure a few either hid or came back when they had the chance. But, they never had the blood of Judah or Benjamin in them. And, that is the DNA of the Israelites in Israel today as well as in 721 BC. I don't know how I can be any more clear. Therefore, whether the Book of Mormon is made up or not doesn't matter to your attempt to prove Joseph Smith is not a Prophet of God.