Without notice, the Savior cites Isaiah. As the Savior works these texts, he explains the future history, and then uses the scriptural indication of their fulfillment. When the gospel has been preached to the Jews and they have accepted and prayed to the Father in Jesus’ name, then “shall their watchmen lift up their voice, and with the voice together shall they sing.”
This method of using the scriptures reverses the typical method of exegesis, where the scripture is given, and then explained. What the Savior is doing is presenting the future history, and then punctuating the message with the confirmation of the scriptures. He is not explaining the scriptures, but he is using them as final proof of the truth of his explanation. They become the culmination of the argument, not the beginning point.
Reference: The Savior continues referencing Isaiah 52:
Isaiah 52:8
8 Thy watchmen shall lift up the voice; with the voice together shall they sing: for they shall see eye to eye, when the LORD shall bring again Zion.
The Savior cited Isaiah 52:8-10 on the previous day (3 Nephi 16:18-20). During of this particular discourse, the Lord will cite most of Isaiah 52, but not in order. The fact that it is broken into sections suggests that the Lord presumed that the people would be familiar with the text, or perhaps remember it from the previous day’s citation. The congregation was expected to pick out these verses and understand their combined context as well as this recutting of the text with interspersed comment.