You say "I'm unable to find any historical evidence this statement was ever true." I can only guess that your failure is based on a reluctance to even try, or you would have found a whole raft of ancient cultures and their public intellectuals documenting flat-earth beliefs in the Wikipedia article on "Flat Earth."

I am and was very aware that knowledge about the Earth's sphericity is more ancient than some people may think; so I intentionally made my statement sufficiently vague to avoid your kind of quibble.