The only way to correctly understand this verse is to read it as Jehovah, the God of their fathers. Nephi specifically invokes the Israelite history and declares that the God who led Israel from bondage is the very God that will come in six hundred years. When Nephi declares that this is information declared by an angel, he is most likely referring to his great vision of the tree of life. He might have received the information later, but the subject of that vision led to a discussion of the mortal ministry, and therefore is likely the time Nephi received this information.
Nephi specifically invokes Zenock and Neum, two prophets from the plates of brass. The assumption that the plates of brass reflected a northern kingdom record, rather than the record of the southern kingdom which gives us our Old Testament, provides the most logical reason for the absence of these prophets from our current Old Testament.
The specific information that is added is that not only will Christ’s rising from the dead after three days be a sign in the Old World, but that it would be known by those who were on the isles of the sea. Nephi clearly saw his people as being on the isles of the sea, and he will highlight times when Isaiah also speaks of those on the isles of the sea. It is a theme that we will see in Nephi’s and in some of his brother Jacob’s writings, but which fades when the Nephite nation no longer has members who came from across the sea.