You would think they would know what a gunshot sounds like but maybe with everybody yelling etc...
They've probably never heard one without ear-pro.
Not trying to make excuses for the cops, but I know that recognizing gunshots (either incoming fire, or people shooting at other people nearby) can sometimes be difficult. I've seen folks in a combat zone look at each other for a couple shots thinking "is that gunfire?".
I bet those cops were also primed by their higher to expect that evil racist MAGA crowd to be armed to the teeth, so they were expecting gunfire and that's what they interpreted loud sounds to be.