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.
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.