When photographing, if you want to get separation between your subject and background, meaning, you want an in-focus subject with background objects blurred, you open the lens aperture as wide as possible.
Think of lens apertures as akin to the pupil of an eye. The wider the aperture, the more light is let in and vice-versa.
My understanding is he had the lens aperture at f1.6, or f1.8 which is the photographic equivalent of getting your eyes dilated. To not overexpose the image, the shutter speed would be cranked up to a high speed in order to reduce the amount of light reaching the sensor and not over-expose the image.
Furthermore, a high shutter speed would work to the experienced photographers advantage in that should something dramatic suddenly happen... a large bird flying overhead craps and hits the subject square on the face, the photographer could freeze the action and get a shot news organizations around the world would eagerly pay for.
So, on the surface, I don't see anything out of the ordinary for the settings a professional photographer might choose in that setting.
Ultimately, by the time the photographer heard the shot, the bullet had already long passed by. No way the photographer could have timed that shot. It was a once in a career moment.
Trump gesticulates at these rallies and if the photographer wants pics where his subject’s hands and arms aren’t blurred as he moves, his camera would be set for achieving that effect. Whether those particular settings would’ve captured a speeding bullet, I have no idea.