This photo was taken well after sundown. The light on the ridgeline is reflected from snow off the Olympic Mountains, bouncing back into the sky and reflected again off the clouds. It looks as if it could be from city lights, but there is no city in this direction, just mountains for a hundred miles or so. 
The photo is also an excellent demonstration of the low-light capabilities of the iPhone X. The human eye can distinguish many subtle variations in light and color, and cameras are not yet even close to such sophistication. But they are making astonishing strides, especially when backed up with the computer power of a sophisticated phone.
