There is nothing to suggest that Isaiah thought the idols were real gods. Isaiah very clearly believed in only one true God. But he is also not simply ridiculing them. He uses poetic language to describe the coming of the Lord and the destruction of the idols. Egypt will feel the judgment of the Lord, and they will shake with fear; their idols powerless to save them.
1 An oracle concerning Egypt. Behold, the LORD is riding on a swift cloud and comes to Egypt; and the idols of Egypt will tremble at his presence, and the heart of the Egyptians will melt within them.