But that's really only because the paper size is measured in inches, and it allows us to know how much of the paper area will be filled. But a video screen is measured in pixels, and the video system only knows how large the image is in pixels. Knowing the image is 600x400 pixels tells us a lot about how it will fit on a 640x480 pixel screen. But to complicate things, not all screens are 640x480 pixels, same as all paper is not 8.5x11 inches. The book version goes deeper into this, elaborating why other details like the so-called 96 dpi fonts in Windows don't affect images, and why screen captures that appear to show 72 or 96 dpi are not what they seem, but the main point here is that monitors vary, and they are obviously not 72 dpi.
Screens work only with pixels, there are no inches and there are no dpi on the screen video system.
For sure don't believe that. It is the world's worst imaging advice. You only have to test it once to know it's wrong . On any one monitor, we can easily see that there is obviously a tremendous difference in viewing the same photo scanned at 72 dpi and at say 200 dpi. The 200 dpi image is much larger on the screen, about 3 times larger.