It is not clear to which part of the scriptures Abinadi is referring when he proclaims Moses as prophesying the redemption of Israel through the Messiah. The modern Book of Moses is certainly clear on this point, but those are sections of the Book of Moses that do not correspond to our received text of Genesis. Perhaps there were some differences in the brass plates and these included more specific prophecies from Moses that we do not now have.
Polemics: It is fascinating that it is Moses that is mentioned, and not Isaiah. Abinadi is going to cite Isaiah, but he first references Moses. The rhetorical reason for so doing is obvious. He has spent time showing the gospel of the Messiah as superior to the law of Moses. He clinches the argument by declaring that Moses himself knew this, and prophesied of this Messiah that the priests deny.
While this is a tremendous rhetorical ploy, it is a poor one for one who is writing the Book of Mormon as a novel or a fantasy. Here would be an easy place to submit a proof text that is accepted (such as the Isaiah citations that will come). However, rather than the safe entry, we have this odd one that would appear to be deniable from the current Bible. A good imposter would have avoided this argument, even though it is powerful. In a true ancient work, however, Abinadi could easily have access to information that is not in our current version of the scriptures.