The book of Genesis is an abbreviation of the lives of the Patriarchs. As with so many other instances in the Book of Mormon, we see that the Nephite record was more complete than our own. Moroni was quoting scripture to rally his people. In his version of the life of Jacob, the above prophecy was included. According to our Bible, the only thing the torn garment of Joseph represented was his death by wild animals (Gen 37:32-25). In Moroni’s Bible, the garment represented Joseph’s posterity which would be forever preserved.
Hugh Nibley
“The Nephite prophet Moroni tells a story, which he says was common property of his people, concerning the death of the patriarch Jacob (Alma 46:24-25). I have never come across this story except in Tha’labi—who in Joseph Smith‘s America had access to Tha’labi? Tha’labi, a Persian in the tenth century A.D., went about collecting old stories of the prophets from his Jewish neighbors. The story in barest outline is that when the garment of Joseph was brought to Jacob on his deathbed, he rejoiced because part of it was sound and whole, signifying that some of his descendants would always remain true; but he wept because another part of the garment was befouled and rotted away, signifying that part of his descendants that would fall away. The same story is told with the same interpretation in Tha’labi and in the book of Alma, in the latter significantly as a popular folk-tale.” (The Prophetic Book of Mormon, p. 249)