The article was saying that this was likely the first shot. Standard bullets fired from an unsuppressed AR-15 travel faster than sound. No one in the crowd would have heard anything until about 70ms later (shooter about 125 yards away, assuming standard 62gr cartridge). Trump would have heard the crack of the bullet flying past him and maybe a few others close by but the rest of the crowd would have heard nothing. Based on how close that streak is to Trump, I doubt the photographer would have heard anything at the point the first photo was taken. Very rare to capture a bullet on camera when you don't even know it's coming.
Interestingly, Trump's statement after the incident seems to support this. When I initially read it I thought it was odd that he noted the whizzing first, but it now makes sense.
"I knew immediately that something was wrong in that I heard a whizzing sound, shots, and immediately felt the bullet ripping through the skin"
