Just because changing from a focused view to the peripheral view makes the spinning ball “curve,” doesn’t mean that a curve ball is an illusion. But this visual illusion makes clear that changing your primary focus can create an illusion of curving, and it occurs to me that maybe this explains why some hitters have such problems with the curve.
Keeping your head still during the swing is one of the tenets of solid hitting fundamentals, but it isn’t easy. This would also have some bearing on fielding fly balls on the run. Or it’s just a neat trick among many.