To me, it’s: That ancient people thought the Earth was flat.
We have records from around 430BC where Greek philosophers spoke of the Earth being a sphere. In 240BC the Greek astronomer Eratosthenes calculated the circumference of the Earth and was only about 2% out.
In general that old math theorems/ideas are named after the mathematicians who discovered them.
Pythagoras didn’t discover the Pythagorean Theorem. Pascal didn’t discover Pascal’s Triangle. Fibonacci didn’t discover the Fibonacci Sequence.
My curiosity was peaked so I googled for anyone else interested:
The Egyptians.
Islamic mathematician Abu Bekr ibn Muhammad ibn al-Husayn Al-Karaji.
described by Indian mathematicians as early as the sixth century