They say "thus, on average, about two-thirds of cats preferred to sleep on the left side of their body with their left shoulder down", and their image for leftward lateral bias shows this. So I guess leftward means "lying on their left side", not "curling left".

But, they suggest this is because "Upon awakening, a leftward sleeping position would provide a fast left visual field view of objects", which seems suspect. When my cats sleep on their left, it's their left eye that's obscured by their paw, and their right eye that has a better field of view!