Isaiah 8:21 starts a new speech concerning the new reality, that is, the period that follows the Syro-Ephraimite war. The division of the speeches does not follow the division of chapters. The division into chapters to which we accustomed was first taken over in the 14th century. For the scholarly methods of determining Isaiah’s speeches, consult my article “Reflections on the Study of the Prophetic Discourse,” Vetus Testamentum 33 (1983), pp. 207–221.