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.
It depends which ancient people. That may be what thought in Greece, but what about elsewhere? Also a lot of Greek and Roman knowledge was lost by the dark ages or ever made it to Europe at all.
Knowledge of a spherical earth was never lost amongst serious academics in post-Roman Europe.