They also knew there was a large land mass in the north and that’s why he took the southern route to try to go “under” that to the east.
Yes, they knew the latitude of the places they wanted to get to so it was inevitable they would do that.
Columbus had no idea how big the Earth was. But a Greek mathematician a millenia earlier had come pretty close to the actual circumference.
https://en.wikipedia.org/wiki/Eratosthenes