I think I understand the author's idea.
The guard can be generated correctly, or it can be generated incorrectly (shifted). If there is no normal guard and no shifted guard, you can freely pass through.
If I understand correctly, IMO it is better to correct the item because the interpretation is ambiguous (see my previous post).
"Absence of a normal guard" or "absence of a shifted guard" is the same thing: "absence of a guard." The item will be shorter and clearer.
5.6.3. In the absence of a guard, free passage from one zone to another is allowed.