Looking at a different piece of bystander video of last night's terrorist attack in east London, it looks to some of us as if the cry of "You're no Muslim bro'" was directed at the policeman handcuffing the perpetrator, who has a neat beard and a Mediterranean appearance, not the man who was keen to smite at the necks of infidels in revenge for Syria.
The police might want to talk to the person shouting at this time, a young man with a distinctive beard just in shot at the right. His other remarks were "drop it, drop it". I suspect he may be aquainted with the attacker. Not necessarily well acquainted, but enough affinity to consider him a brother, and to know that Islam was a factor. The local mosque is the Cann Hall Deen, which took over the Colegrave Arms pub. I bet they have never seen this young man before, no matter how many times he attended!