This is an old revision of the document!
Isaiah 1:1
Either he started writing all of this after the reigns of the kings, Isaiah didn't write it, or he prefixed it later. See also Isaiah 2:1, which appears in the Book of Mormon. In Isaiah 6:1 and Isaiah 8:1 he speaks of himself in the first person.
Isaiah 1:19-20
He gives his testimony, after the prophecy, that it is of the Lord.