The drawing is maybe off (problems with rescaling) but the measurement notes are the right ones, so this shouldn't be a problem.
The example is all about how the box-angle changes if you change the viewing angle and this is the correct part.
There is no way to make a correct axonometric isometric box, because you only can achieve the 120° wide angle with a viewing angle of 35,2° you need for isometry. The axonometric perspective you use (or the one in the example) has a viewing angle of 45°. So no you can't make an isometric looking and flat at the bottom placed box for a 45° viewing angle.
The correct constructed 45° axonometric rpg perspective box is the one with the jaggy lines, but I already pointed out, that this isn't very beautiful for pixelart standards.
Try to understand the example, if it's to hard to understand, try to imagine the projections first.
The example illustrates all things you need for understanding.
The good thing about the human brain is that you only recognize perspectivical, proportional or any drawing-related issues if you have a lot of viewing-experience. Most people aren't well trained to recognize this, so it's pretty easy to fool there impressions. The 13° deviation of the viewing angle won't hurt the impression of most people.
If you thought a lot about it and you weren't able to understand it you can send me a msg. Atm I don't really have lots of time, which explains also why I haven't completed all so far.