We see the lady drop her phone and the man with the camera turn towards her. They are then out of view and when we see them again she is behind him and he is turning as his phone is falling to the ground and she is turning to walk away. You would struggle to prove assault from that. I would guess she knocked the phone out of his hand. The police officer is right to prioritise the protestors moving along the street, than a disputed, minor, assault.
You really don't need to describe what's going on, especially when you leave bits out that don't help the way you've coloured it in. Like this bit where
we see the male walking along the road not looking at her at all. She is on his left, he is looking to his right.

Or this bit where
we see that she has moved across to his right and is saying something to him. There's a 3 sec gap between the first frame above and this one. That'll be the point, for some, where the guy who was ignoring her 3 seconds ago carried our some heinous action which prompted her to approach him aggressively.
Below is the point where she has dropped her phone and is picking it up. She's got the males attention and he's stopped and is filming her. I'm betting that that is what caused her to kick off.
Less than 1 sec later something causes this guy to turn around and look in their direction.

Within a couple of seconds here, below, is another guy turning to see what is going on, can you recognise him? That's right, it's officer, "go away", who said he didn't see anything. Lying bastard.

Here
we see that male has turned away from the girl and has his camera in both hands. Her elbow is just in the frame. His elbow is raised and it
looks like he is pulling away from her as he leans to his left. This is right after she assaulted him.
Here
we see that she is still moving towards him and not slowly as her hair indicates. The assault has already occurred at this point.
Just filling on some gaps for you. Isn't it great what
we can see?
In my time, at football matches and marches, I had people complain about alleged abuse, fights and assaults, and it was clear at the time, what ever had happened, if anything, was exaggerated and appeared to be intended as a distraction.
Clear how?
The bottom line is that the police often do not have enough resources at such events, so that they can lose officers to minor crime or incidents.
Not Joe Public's problem. "Sorry sir were you assaulted? Due to our lack of resources, even though officers were only feet away, you can "go away".
Then, when they do have the resources, they can get criticised for over reacting, depending on whose side people are on.
Maybe they shouldn't over react then? Never mind whose side people are on, if their behaviour mirrored the articles of their oath of attestation, swearing to serve the Crown and act with impartiality "without fear or favour", such accusations wouldn't exist.
It is clear that the officers involved here decided to ignore the assault.
What about taking a second to have a quick advisory with the woman, or even both of them? Both of them are in the march, both of them are involved in some sort of infraction, both of them were involved in an incident that could have inflamed things.