If the photo was really taken at 1/8000th of a second shutter speed, then the streak in the image had to include “mist”, as you mentioned, or the bullet’s shock wave vapors. A short (one inch or so) bullet traveling 2000 fps would leave only a 4” streak at that shutter speed.
Since the streak appeared to be maybe 12-18" long, shutter speed was considerably less. Halving the shutter speed to 1/4000 would essentially make the streak 8 inches long at that particular bullet speed. I would venture to say that he was shooting in the 1/2000 to 1/2500 second range on such a nice sunny day, with the aperture at f/8 - kind of a middle-of-the-road aperture setting. ISO probably around 400 to allow for faster shutter speeds.
If you can get the original image file, the camera settings will be included in the EXIF information.