He definitely choked him from behind, it's as clear as day on video. He also had it very deep and as a trained Marine, he definitely has it tight and knew exactly what he was doing. His other hand was on top of the back of the head, which is used to secure a choke and make it tighter. So to hold somebody like that for 6 minutes you are definitely going to cause damage.
I once had a scuffle where I ended up in a similar situation, except I let the choke go after about 20 seconds when he went limp. If I held that for 6 minutes, I would have killed him and probably gone to prison.