We don't have to answer the question of whether it was a strike
This was a direct claim from you. You did not even present it as a possible scenerio, you said she striked him in the face, multiple times. Is this something that you think is visible from the video? If not why did you make this claim?
You have conceded that she used force,
Nope, all I have said is that she did not strike him in the face and that her movement towards him could be anything including a shove. I wouldn't even consider it a lunge, so as it stands, it is a very hard sell for reasonable grounds for suspecting harm. Nevermind proportionality aspect of kicking the woman who may or may not have tries to push you. And this is something you have been glossing over.
If you can't conclusively prove she was harmless
At this point we are acting only on suspicion. I don't think one can conclusively prove that she was initially trying to be harmful either. But lets assume that he really suspected that she meant physical harm and pushed her. At that point, until she gets up and tries to attack (assuming that was her intention to start with), the threat is assumed to be paused. Otherwise what is the difference between one kick and ten kicks? What you defend suggests he should be able to kick her as long as she tries to get up which is open to abuse. So kicking at that point is considered retaliation and it is your duty to retreat if you are in a public space and not your home.