Not a flat earther here, but a flat earther would argue that Eratosthenes’ observation has two solutions: A round earth, or a small sun close enough that it’s rays were not parallel. A small sun ~6300km away would give the same result as a round earth for the observed values.
I’m more inclined to argue using the stars. If the earth was flat, why can’t we see the southern cross from Europe? Why does the sky seem to rotate around polaris in Europe, but around a point near the southern cross in southern Africa. That simple observation is entirely incompatible with a flat earth.