Well, I can't view the video so I am at a disadvantage. But could it be that someone other than the Muslim muttered OMG, and everyone thinks that the killer said "allah akbar"?
Unlikely as the voice was distinctly different from that of the officers and clearly sounded foreign. having listened to it over and over, and having read other possibilities and then listened to it over and over. My verdict is he shouts [Allah Akbar! Allah Akbar! Allah! gunfire]