Or look at the video preceding this incident, and you'll see that the officers ran ahead of their group, and the kids confronting them were not looking at their phones because they were busy trying not to get shot.
Or come and see for yourself.
Or rely on the eyewitness testimony of hundreds of journalists in Hong Kong who have seen this same situation play out—cops run out too far, get isolated, get scared, draw a sidearm.
Or figure it out from first principles on a message board.
Are you saying this video is taken out of context? Please do share any available video evidence. I assure you I tried quite hard to try to find more and this all I came up with! This is the thing that most confuses me about these protests. They are, by far, the most widely recorded protests in a developed nation - yet lengthy in context footage of the critical events events is surprisingly difficult to come by. Instead we just get snippets taken out of context that can be framed to show anything you want to show.
For instance this [1] is Reuters reporting of this video entitled "Dramatic footage captures moment when Hong Kong protester was shot." It strips out absolutely all context and splices the video together in a way that even makes it seem like it's an officer charging a protester who is simply trying to defend himself from an officer drawing a gun. It then includes clips from a police statement which, without the context of what that conflict was about, paired with the selected footage they provided sounds comically absurd.
This is absolutely propaganda, and quite ham handed propaganda at that. But of course there is also extensive propaganda coming from the Chinese side as well. It makes it all extremely difficult to have any clue of what is really happening. If you have any recommendations whatsoever on lengthy uninterrupted film footage on these events, I am absolutely all ears.
[1] - https://www.reuters.com/article/us-china-anniversary-hongkon...
There's also lots and lots of long-form protest footage available, but you need a Chinese speaker to help you Google your way to it.
I don't speak Chinese or have links handy but it is not hard to find with some help from a native speaker. Dig around LiHKG.
You could make an argument that somehow they used some other technology to carry out their attack, but I'm not sure how much weight it'd have beyond the fact that no we don't have video (to my knowledge) of one of those guys explicitly opening and looking at the app to find the target. In any case, it certainly refutes the argument that no protesters have managed to catch the police by surprise or ambush them - because that is exactly what happened.
https://www.youtube.com/watch?v=PN4MvOrPotk