These verses were also quoted by Nephi (1 Nephi 21:24–26) and are about the covenant people of the house of Israel being delivered from the Gentiles. The bolded text (v. 17) is a comment inserted by Jacob for emphasis. Nephi interpreted the message to be the Lamanites in America being delivered from the Gentiles. Jacob is speaking of the Jews being delivered from the Gentiles. His commentary comes later in 2 Nephi 10.
Jacob continues reading from the Isaiah text included on the plates of brass, what is now Isaiah 50–52:2 in the KJV. There are many significant retentions in the Book of Mormon which help us understand these passages. Modern revelation confirms their interpretation.