We can agree and disagree on this one depending on perspective. i agree that he was probab ly holding his phone when he was shot, however others may have been unsighted.
He was on all fours when the shooter took the decision to pull his gun. the other LEOs then disengage. at the same time another LEO takes his gun. APs hand goes down and to the right, which is towards his holster area and the shooter fires.
It looks like the shooter could a) have possible have seen that the holster was empty, but APs jacket was flying around and b) not been able to see APs right hand clearly due again to the jacket. But maybe shooter had already opened fire or taken the decision to open fire and these two points are irrelevant - probably.
In this confusion when you cant see his hands, when he is acting violently, when you dont know where the gun is but you know there is one - boom.