Lehi explains the particulars of the promise to Joseph of old. The lineage would produce a righteous people, but was not directly in the Messianic line. The important point for Lehi is the "branch which was to be broken off", for that once again highlighted his sons as the bearers of this tradition and blessing (and yet again argues for the command of the Lord in their leaving, a subtle comment for Laman and Lemuel).
It is interesting that Lehi does not expect that his descendants would be righteous throughout their existence, however, for he specifically notes that the Messiah should bring them out of darkness to the light. This clearly presumes a falling away at some point, before their ultimate redemption through the words of the Messiah.