"This assurance of the final fulfillment of God's purpose brought courage to the heart of Isaiah. What though earthly powers array themselves against Judah? What though the Lord's messenger meet with opposition and resistance? Isaiah had seen the King, the Lord of hosts; he had heard the song of the seraphim, “The whole earth is full of His glory;” he had the promise that the messages of Jehovah to backsliding Judah would be accompanied by the convicting power of the Holy Spirit; and the prophet was nerved for the work before him. Verse 3 . Throughout his long and arduous mission he carried with him the memory of this vision. For sixty years or more he stood before the children of Judah as a prophet of hope, waxing bolder and still bolder in his predictions of the future triumph of the church. PK 310.1"
"Then said I, Woe is me! for I am undone; because I am a man of unclean lips, and I dwell in the midst of a people of unclean lips: for mine eyes have seen the King, the Lord of hosts."
The EGW passage references Isaiah's vision of God and the seraphim, and connects it to his courage and mission. While not a direct quote, it's a clear reference and thematic parallel to Isaiah 6:5 and the surrounding verses.