Historically, the black flag is strongly associated with anarchism, anti-state politics, revolt, and rejection of national authority.
Had he colored it in the union jack, then I would've said it was nationalism, and the person is blinded by nationalism.
But. This is Banksy, black-and-white Banksy, so there may be no symbolism behind the black flag, but it's just very interesting. I can't accept that he would not have considered the color of the flag.
But from an American perspective a guy wearing a suit while carrying an "anarchist" flag wouldn't be inappropriate, either.
We anarchists with careers do in fact exist. There are probably dozens of us outside of tech, even!
If I had to ballpark it, I’d guess something like 1:5 people in tech are broadly aligned with me politically (meaning “less extreme, but directionally similar”) while maybe 1:100 would self-identify as an anarchist and 1:500 both self-identify and align fully with me.
Does that help?