Actually, they _are_ probably using multiple really freaking bright flashes, or rather flash bulbs. There are still some around that blow away anything else you can get down into a cave for a power/size ratio. Think streetlight sized bulbs filled with magnesium.

There's probably some light painting going on as well with (bright) flashlights and long exposures.

The 1/r^4 really only applies if the light is coming from the camera position, and is unfocused. If you're illuminating the subject from another location, the only worry you really have is the 1/r^2 from the source to the lit object.