Isaiah said this because he saw Jesus’ glory and spoke about him.
Morris speaks of the complexity of Christ’s glory thus, “It points at once to the supreme greatness of Christ and the cross as the supreme illustration of His greatness. Here it included the thought of His rejection, for that, too, is part of His real glory. He being what He is stooped to a position where men might and did reject Him.”
Isaiah “saw” it. This may be speaking of a “seeing” by faith (8:56, Heb 11:13) or when Isaiah actually saw the glory of Yahweh (Isaiah 6:1-3). It is very possibly the latter (1:14).
John 8:56 Your father Abraham rejoiced at the thought of seeing my day; he saw it and was glad.”
Hebrews 11:13 All these people were still living by faith when they died. They did not receive the things promised; they only saw them and welcomed them from a distance. And they admitted that they were aliens and strangers on earth.
Isaiah 6:1-3 In the year that King Uzziah died, I saw the Lord seated on a throne, high and exalted, and the train of his robe filled the temple. Above him were seraphs, each with six wings: With two wings they covered their faces, with two they covered their feet, and with two they were flying. And they were calling to one another: “Holy, holy, holy is the LORD Almighty; the whole earth is full of his glory.”
John 1:14 The Word became flesh and made his dwelling among us. We have seen his glory, the glory of the One and Only, who came from the Father, full of grace and truth.
Isaiah was told this prophecy by God so that he would tell us (Isaiah 6:8-10). Isaiah always passed onto us what God told him to say (Isaiah 21:10). Are you willing to be used by God in this way?
Isaiah 6:8-10 Then I heard the voice of the Lord saying, “Whom shall I send? And who will go for us?” And I said, “Here am I. Send me!” He said, “Go and tell this people: “‘Be ever hearing, but never understanding; be ever seeing, but never perceiving.’ Make the heart of this people calloused; make their ears dull and close their eyes. Otherwise they might see with their eyes, hear with their ears, understand with their hearts, and turn and be healed.”
Isaiah 21:10 O my people, crushed on the threshing-floor, I tell you what I have heard from the LORD Almighty, from the God of Israel.