They think the additional records of which Nephi spoke are the various ancient manuscripts related to the Bible. It's true that ancient manuscripts have been found that corroborate and/or tend to prove that our current Bible is a reasonably accurate transmission of much older copies. But those manuscripts don't add to our knowledge; they just confirm the accuracy of the text.
We know from our modern prophets that additional Nephite records will yet come forth. They were stored in Mormon's depository in the Hill Cumorah in New York before Joseph, Oliver, and other brethren moved them.
But they will yet come forth.