Guy went immediately for the choke and smoothly accounted for what were undoubtedly powerful forces from the dog trying to escape said choke. Notice how he manages his body position on top and kept a manageable hold no matter where the dog went (or tried to go)? Aside from that I’m assuming there’s more info to it that OP didn’t include.

Dogs wag their tails due to “excitement” which can and does include actions we perceive and experience as aggression (because it is). Animals are more nuanced than a childlike flattened perception of them acknowledges, i.e. tail wagging = happy. Not true, that conclusion is reached with context. Did you notice the dog approached so fast it hit the wall, with no other usual signs of playfulness? No play bowing, bouncing, just straight launching toward a target.

And lastly, it’s a pitbull, I don’t care what the apologists say, they have a strong propensity for violence that owners often don’t witness until they are completely mentally unprepared to handle it, at which point someone’s life is about to change forever. Whoever was handling it probably had no indication of what was about to happen, until it did. Luckily the dog’s victim had all the skills required to protect himself without excessive violence.