That explanation irks me a lot because it implies Eratosthenes knew the Sun was so far away, the rays reaching the earth are practically parallel. And while it

That explanation irks me a lot because it implies Eratosthenes knew the Sun was so far away, the rays reaching the earth are practically parallel. And while it might sound a like a detail, the whole calculation depends on it. If you presume the Earth is flat and the Sun not that far away, Eratosthenes was calculating the distance to the Sun.