[blocko]

They mention this under the "What if there is no true perspective?" section:

Second, they point out that viewers almost always view pictures from the “wrong” location. A cornerstone of linear perspective is the idea that the viewer must be at the focal center of the image to view it correctly. If you view a linear perspective image from the focal center, it should be like looking through a window—indeed, Leonardo da Vinci wrote that linear perspective images only work from the focal center. Yet, in reality we don’t do this. Most of the photos we look at would have to be viewed with one’s eye a few inches from the page. People in art galleries walk all around and view paintings from all sorts of angles. Hence, the whole idea of linear perspective falls apart.