Jacob now gets to the particulars of the beginning text from Isaiah. The issue of the text is not simply the redemption of Israel, but the redemption through the gentiles. Jacob makes this very clear. In verse 12 he continues his own exegesis. The phrase "they of whom the prophet has written" does not introduce the next clause, but is a subphrase of the first. Jacob is restating that it is the Gentiles that are explicitly mentioned in Isaiah.
The second clause is Jacob's explanation - clarifying that the Gentiles will be the means of achieving the blessings for Israel, upon the conditions of their righteousness.