But that issue is related to the fact that insects have their nerves in the inferior part of their bodies, and the digestive tract on top, while we have our nerves in the back (protected by bones), and the digestive tract on the front, which would be the lower part if we were still walking with four limbs.
The issue described here is left-right symmetry, and it also applies to insects, so it is more general.
So far I have read that these are related theories and the "somatic twist" theory is an expansion on the earlier theory of inversion. And there seems to be another related but separate theory called the axial twist theory (explained in the linked paper).
The issue described here is left-right symmetry, and it also applies to insects, so it is more general.