Defensive positions are on p. 11 of the rulebook. Page 12 is only about buildings.
The rule you are referencing actually reads:
Some obscure LoS. In this case, the entire layout obscures LoS.
It does not say EVERY Defensive position obscures LoS, only that some can, and if they do then the entire layout obscures LoS.
Apparently they had some sort of object in mind, but atm I am not up to digging through the entire collection to figure out which overlay (if any) may fit that bill.
However, there are several other bits in the rules that are not all found in the core box. They may have been items originally planned for the core box but later reassigned to an expansion board during development or perhaps were always planned for an expansion, but it was simpler to include the rules in the book.
I personally applaud that sort of inclusion, as it is one less rules sheet I need to reference when trying to track down a specific rule in the middle of a game.