Isaiah, is, of course, a major prophet, BUT the office of prophet does not always mean that the words he speaks is future; prophet simply denotes the person who speaks for the Deity.
I know, but the context of the chapter clearly foretells of a future event.
"that day" refers to the future days of "a saviour, and a great one, and he shall deliver them" (verse 20).
This hasn't happened yet.
Look at a little more of the chapter:
In that day the Egyptians will be like women. They will shudder with fear at the uplifted hand that the LORD Almighty raises against them. 17 And the land of Judah will bring terror to the Egyptians; everyone to whom Judah is mentioned will be terrified, because of what the LORD Almighty is planning against them.
18 In that day five cities in Egypt will speak the language of Canaan and swear allegiance to the LORD Almighty. One of them will be called the City of Destruction.
19 In that day there will be an altar to the LORD in the heart of Egypt, and a monument to the LORD at its border. 20 It will be a sign and witness to the LORD Almighty in the land of Egypt. When they cry out to the LORD because of their oppressors, he will send them a savior and defender, and he will rescue them.
"In that day the Egyptians shall be like women", "in that day there will be an altar to the LORD", this contradicts your scenario that there was no Egyptians around when the GP was built.
So, have you got any references to the GP in the Bible?
Brian.