Looking at google earth and it looks like they were lucky it wasn't even worse. The only access to the area circled in blue and the development to its right is that one road running through a rather steep canyon. This is very similar to the Paradise CA fire, which killed 85.
Also of interest is the reservoir that was empty is circled in red. The only area that was not heavily damaged was the area in blue! The area to the right of it was flattened and at a higher elevation, so that reservoir would be no help to that area. (I think the fire may have started up there)
It seems to me, it's only a mater of time before that blue area burns. Get a fire up there, add some strong Santa Ana winds, I think it would be very hard to stop. I hope the people that live up there are aware of how dangerous their situation is. I suspect they are a bit more aware now!
BTW that fire is currently working upwind up the mountain but will need to travel down slope against the wind to get to the rather large amount of houses on the other side. Massive numbers of helicopters attacking it there now (nighttime), so I suspect it will be unlikely to hit those houses.