Even if we set Isaiah aside, the Book of Mormon still refers to Old Testament texts in a way that is impossible given what we know about the Old Testament’s development. For example, when Lehi leaves Jerusalem in 600 BCE, he sends his children back to get the brass plates. Upon receiving them, he finds

that they did contain the five books of Moses, which gave an account of the creation of the world and also of Adam and Eve, who were our first parents; And also a record of the Jews from the beginning, even down to the commencement of the reign of Zedekiah, king of Judah; And also the prophecies of the holy prophets, from the beginning, even down to the commencement of the reign of Zedekiah; and also many prophecies which have been spoke by the mouth of Jeremiah. (1 Ne. 5:11-13)

The ”five books of Moses” clearly refer to the Pentateuch, but the Pentateuch was not compiled—nor indeed was the Priestly source even written—until well after Lehi would have left Jerusalem (see chapter three). There could not, therefore, have been “five books.” Further, the earliest indication that these books were ascribed to Moses does not surface until after the exile.