That's because Reid didn't explain it at all, and just showed a picture that actually could use some improvement. The picture basically tries to explain at what angle the 'camera' is, and... that's it. It doesn't explain much
This one is in my opinion a little bit better. In the top row you see the shapes that you want to draw, and the bottom row shows what you actually need to draw to draw in perspective. Because the camera is at a 45 degree angle, the height gets 'squashed'.
You can see when the coin rotates, the height changes (the top one is very thin, almost no height, the bottom one is the full height).
I hope this explains it a little bit better. Now to draw the correct perspective, all you need to do is multiply the height by 0.7. So if you draw a circle, and that circle is 32 pixels height, and 32 pixels width, to draw in perspective you need to draw a circle that is 32 * 0.7 ~= 22 pixels height. So a 22 pixels height and 32 pixels width.
I hope this explains it a little bit better.