They knew it for the right reason; centuries before, Eristophanes calculated the circumference of the Earth.

He was wrong in the size of the Earth, which is why he couldn't account for a continent in the way. By his faulty calculations, there was no where for the continent to *be*.

Ah, but they knew it for the wrong reason. Columbus was right in that if he continued to sail westward, he would eventually circle the globe. In that he was correct, he merely didn't count on there being an entire continent in the way.