Eratosthenes only tried to measure Earth, assuming a spherical Earth and a very far away Sun, both of which were already known with a reasonable level of certainty by better learned people of the time.
What is not explained by any "FE theorists" is the well known fact, corroborated by most navigators since at least the last 500 years, that traveling 60 nautical miles due North makes all celestial bodies appear 1 degree further South. In a sense, this is Eratosthenes' experiment repeated in all latitudes, by navigators from all over the world, too many times to count. In fact, the sextant, the most important navigational equipment of last millennium, is based on this observation.
A flat Earth and any assortment of hypothesis set out in this forum can explain one, or a few observations like Eratosthenes' but no FE hypothesis yet can even start to explain why this observation, made from any place on Earth, gives the same result ( 1 degree every 60 NM) and simultaneously why the Sun, Moon, planets and stars all have about the same brightness when they are close to the zenith with respect to when they are close to the horizon.