Just looking through that PDF, unlike the chair in the blog, some of that furniture is not as robust. Pages 46 & 47 the load rests on the corner of the wood, and pages 46, 47 & 52 add almost all load onto the screws.
The chair in the blog benefits from essentially having all load bearing done by the wood, any screws or nails would be superficial only. We have several good hard wood chairs here with dove tail joints and spring based cushions - and they are excellent.
The chair in the blog is not robust. It is simple, but not robust. It contains a hinge with an extreem amount of force placed on a small area of wood. It will deform very quickly, changing the angle of the recline. Similarly, the sharp points in contact with the ground will wear/weather quickly, putting it out of level. Rustic and utilitarian, but not meant to last. Imho.
I think you should check your own knowledge before double-guessing Enzo Mari - a designer who did the work and had extensive knowledge of form and materials spanning decades.
This is not an ""argument from authority" but "Chesterton's Fence".
Maybe Enzo's implementation was fantastic, but the problem with not specifying these things is that it's not clear if one or one hundred nails should be used to secure each part.
One thing to observe is that people were lighter traditionally (i.e. prior to 1974 when this was published), and the load bearing capacity of a chair was less important than it is today. Also bare in mind that wood has become far more expensive, and people of today would likely be using less dense wood.
On page 52 for example, each leg (E) is nailed/screwed into C by just three points. The C wood itself is in a strong configuration, but the legs are almost an afterthought. Without any lower support (e.g. as shown on page 56), the nails/screws will eventually be levered out. The actual loading on the leg itself is not great, with three nails/screws seemingly aligned with the grain. The result would be a split that runs along the grain, and that may have been what happened to the left leg in the picture.
Yeah, and not specifying them - as well as not specifying the assembly steps, which for some models isn't obvious; I know it, I build the bed frame on page 24 - is part of the learning process.
"Autoprogettazione" is not a DIY book, or a guide, or a procedure. It is a project to gift common people the insight of what goes into making an object. The result Enzo seeks is not that you end up with another object/furniture in your house, but that you end up with a new appreciation of what makes an object stand up in 3D and, for example, support your weight, or flatter your eye.
If you want to follow step-by-step orders, there was and there is already hundreds of books, and thousands of workers do the same in factories - just executing orders.
The chair in the blog benefits from essentially having all load bearing done by the wood, any screws or nails would be superficial only. We have several good hard wood chairs here with dove tail joints and spring based cushions - and they are excellent.