8:42 pm? It was probably getting dark.
This time of year, it's still pretty light outside -- depending on the cloud cover. Yesterday, sunset was at 8:37 PM in Rapid City.
But, during that transition period it can be really hard to see details on the ground, especially if they were flying west "into the sun".
I'd have to do the math, but it's possible that even though the sun had set on the ground 5 minutes earlier, they were still seeing it, even at low altitude.