> People must understand that we don't know how old the Pyramids are, we are only doing calculated guesses.

We have actual, written history of Egypt going back for thousands of years. They recorded who was buried in each pyramid, both on the pyramids themselves and in their records. The Greeks and Romans themselves have written histories that talk about their interactions with Egypt and corroborate much of what they wrote.

Egypt wasn't some kind of insular backwater, the Egyptians interacted with other states. Sometimes they were a regional superpower, sometimes they were closer to a vassal state, sometimes they were broken up into multiple states. There's over six thousand years of continuous history there, a lot can happen.