Then why state the very next sentance as:
(P20) "All models within a number of inches equal to the aura's number(#), excluding the aura's originator, are affected for the duration indicated."
This means that only the models not the area gains HT5 Obscuring. That Creeping Fog also states "for all models, including this one", is to also give the Stitched HT5 Obscuring.
Nowhere is it stated that the whole 4" aura is treated as an obscuring terrain piece or some such, but rather (as I interpret it) it makes the models in the Aura harder to hit (soft cover), and harder to see over (Ht5).
If they wanted the aoe to be treated as an obscuring base, shouldn't the ability be written differently, perhaps "treat the aura as a piece of obscuring terrain" or some such.
Ofcourse, I might just be looking into this too much, coming from games such as 40k and Warmachine/Hordes.