They posted an image of this capstone with the text "actual capstone in the Cairo museum."

It is a genuine capstone, or pyramidion, from a pyramid, one of very few that survives intact to the present day. It's black basalt, still beautifully polished and inscribed. It's just not from the Giza pyramids, it's from the Black pyramid at Dahshur, tomb of Amenemhat III. This is 12th dynasty, centuries later than the 4th dynasty necropolis at Giza, but it does give us an idea of what the pyramidion of Khufu's pyramid might have been like. Knowing Khufu, his was probably granite. He had a thing for granite, the entire King's Chamber of his pyramid is lined with it.

The article I linked doesn't give any sense of scale, Amenemhat's pyramidion is 1.85 meters tall and weighs 4.5 metric tons. No problem for the mega masons that built the rest of the actual manmade mountain it would rest upon, but still a far cry from a 100 foot tall chunk of gold.