The earth was flat according to many people.
Serious flat-Earth thought was ideologically motivated in the late 19th and early 20th century and bears some interesting parallels to climate change denialism.
Among educated people able to afford and read a copy of Eratosthenes yes, but there were probably a lot of ordinary people that were not literate and were never exposed to any actual evidence to the contrary，who believed it was flat.
Flat earthers in the modern world really do have no excuse, education in the basic geometry and simple tools they need to prove it themselves is near universal in the Western world, but back then it was perhaps more understandable
who were the ones capable of funding the project in the first place