This, for me, encapsulates the dilemma of lone police officers. The officer cannot control two people at once, and has to make decisions very quickly. He can't call for back up, because he's fully occupied trying to restrain a dangerous master criminal suspected of crossing the road other than on a pedestrian crossing. This isn't about jaywalking laws and personal liberty, though.
Any police officer is trained to be paranoid, on either side of the pond, and for good reason. You just don't know what people are carrying, and being surrounded by an angry crowd is extremely dangerous.
So the officer did what he clearly felt necessary in the circumstances as he perceived them. He had the first girl's arm in a swan neck hold -- holding the forearm vertical and pushing the hand inwards towards the elbow. Another person intervened, and he punched her in the face. I make no comment as to that -- he was there, essentially fighting with two people. I have the luxury of watching it on the internet, with a slo-mo replay.
The real point of this story is that the Seattle Police Department who sent him out alone have accepted that their policy has consequences, and have supported the officer, by saying it's up to him what force he uses. Metropolitan Police Service, take note. If you pursue single-crewing, one of your officers will end up being filmed using Home Office approved distraction technique number 15 -- a punch in kisser. When that officer ends up on Youtube, please have the courage to support their actions in preserving their personal safety, caused entirely by your policy.