what reason is there to believe this
What part are you disbelieving? That the cop was injured by a black man, or that the reason he didn't use force was to avoid race controversy?
If you're arguing that he wasn't pistol whipped by a black person, that's ridiculous. It's verifiable. They have someone in custody for the crime and it happened in public where people took pictures of it.
If you're arguing that "I didn't use force because I didn't want a race controversy" was a post-hoc addition to the story to stir up media attention, then maybe it was. But Occam's Razor dictates that the reason he was battered and didn't fight back or shoot him was probably because of the reason he said it was.