In keeping with Nephi's tradition of using Isaiah to underscore his own people's needs, Jacob is also applying Isaiah directly to his people. While most would see prophecy as being in the future, for Nephi and Jacob (and therefore the to extent that their people believed) they saw the Nephites as the direct inheritors of these prophetic utterings, and as living in the times of the fulfillment of many of them.
Historical information: Whatever the political and social makeup of the Nephites, and I have suggested that it must have included additions from other populations to create both the explicit numbers as well as the numbers implicit in the descriptions of social organization, it is clear that all of the population was considered in some form to be inheritors of the blessings of Israel.
Presuming a large influx of population, this population would not only have submitted to Nephi's political rule, but also to the adoption into the house of Israel -through which adoption these prophecies Jacob will expound become relevant.