I never learned anything about D shaped hair. I was told that curly hair was oval shaped, as was the papilla where the hair is produced. The more elongated the oval, the curlier the hair.
Never studied hair in any depth.
I mostly have fought a loosing battle against the elements.
It gets wider and shorter depending on the humidity.
The oval shape is symmetric. There is no reason for curl to occur, because any particular direction out from the center of the hair is the mirror image. An oval may be easier to curl artificially, but natural curl requires asymmetry.