A prohibition directs avoidance of an act, but legal analysis may distinguish a defect intrinsic to the act from one connected to an external circumstance. The textbook uses this distinction when discussing whether certain prohibited transactions can still produce legal effects.
“This act is prohibited” and “this act produces no legal consequence whatsoever” are not necessarily identical claims. Especially in transactions, the specific doctrinal conditions must be examined; this conceptual exercise is not a ready-made judgment on an actual case.