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.
Ancient peoples DID think the Earth was flat.
The conception of a spherical earth was only widely accepted in academic traditions derived from late Greek philosophy and even in those cultures, had a mixed reception in popular conceptions of the earth’s shape until the 16th century.
The question is not if they did or did not think, but if what they thinked was backed by historical facts at the time.
Yes. It was backed by the oldest book of events at that time.
There were mathematicians that wrote against Galileo, and a notable one, a Dominican I think.
Everything in the past moves to the category of belief.