I'm surprised Thorfinn is portrayed as not knowing that the world was round.
Contrary to the modern myth most scholars and sailors in the Middle Ages knew that the Earth was spherical and even knew its approximate circumference. The ancient Greeks were the first to accurately calculate it's circumference.
The idea that the world had an edge was an old joke to play on landlubbers, and certainly wasn't believed by seafaring groups like the Vikings.
You can read more about how the modern misconception that people thought that the world was flat got started, here: https://en.wikipedia.org/wiki/Myth_of_the_flat_Earth