Any Greek sailor would have known the sea curves towards the horizon and that distant ships, and land itself, sink and eventually disappear below it as one gets farther away. So they would have known the Earth was not flat. A shepherd, up the hills, perhaps not.
One does not judge the knowledge of a civilisation merely by what the man in the street knows. For instance, a wire carrying an electric current at right angles to a magnetic field experiences a force at right angles to both. That's the principle of the electric motors we have all around us. But the average Joe Blow won't know that, if you stop him in the street and ask. Does that mean we, as a civilisation, don't know that?