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.
You’re forgetting one crucial detail about Eratosthenes’ experiment: the measurements were taken at the same time, noon on the summer solstice, the time of minimum shadow length in both locations.