All depends what "shallow fog" means in this context. The correct definition is " low-lying fog that does not obstruct horizontal visibility at a level 2 m (6 ft) or more above the surface of the earth." In other words it is less than 2m deep, possibly less than 1m deep. Hardly a good reason to pass the runway by, particularly as the runway would have been clearly visible from altitude through such a thin layer.
The fact that vertical visibility info was unavailable tells you that the fog layer was not deep enough to cover the sensor.