From the film I think that it's possible the policeman misheard him (though was perfectly clear on the film that he said 'tea').
The drink driving thing came out of nowhere on the film, and although we don't see the hours earlier to know what happened, it did seem out of the blue.
What confuses me most is why the policeman didn't say why he jumped to the accusation of drink driving. He didn't say 'I saw you' or 'you were seen at blah time etc' he just leapt into making another allegation. It's this combined with his determination to 'win' that makes me wonder about what was happening.
It escalates from 'stop filming', to 'get out of the way' to 'have you been drinkimg' etc with barely a switch in pace and it all feels a bit too pat.
The hat & gear in the mans photo seem to designate him as someone that is there to film in case of trouble, so the police may have pigeon holes him as an agitator? Obviously not ok.
Basically, it's not a clear case of police brutality/ set up, but it all feels a little off and like the policeman has a point of dominance to prove for some reason. I don't see his colleagues as doing anything wrong though after the policeman starts questioning. They are standing by what the main policeman says but, well, they have to given no evidence to the contrary don't they? They aren't close enough to hear all of it so I'm not sure what they can do except go through the procedures.
I could speculate all day but here's a few:
Scenario a: man is 'known to be one of / associating with' the protestors (or assumed to be) and therefore police feel justified in being ready to up the ante or find an excuse to confront him. Not ok.
Scenario B: man has been around driving and smells of drink and that's why policeman is so keen to up the ante. Still not really ok but I sort of see it unfolding like that when everyone is tense & tempers are raised. Related thought, should police be in better control of their tempers? Or is adrenalin difficult to regulate in these situations? And are we ok with that excuse? Meh, I can't really justify it.
Scenario C: police are so riled up by real/ imagined taunting/ violence/ ruckus that they are just waiting for someone to start it, lots of adrenalin, maybe fear, maybe fun of fighting... Again, not ok but I can see it happening. Again it's the adrenalin thing.
I suspect the enquiry would hang on the smaller evidential details: whether the car mentioned was really the guys car, did the mans breath smell of alcohol? Was anyone else aware of the car driving ...
And the real truth would depend on knowing whether the policeman had that attitude on him to everyone, or was calm and fair all the rest of the time, or whether he's a bully who uses his official status to dominate others and gets a kick out of if.
Unfortuneately we can't answer any of that!