The taking of the knee is to show solidarity against the policeman who murdered George Floyd, the fans here are fed up with it because it's getting in the way of their enjoyment of the game, they don't want their game politicized imo - how long is it going to go on for? They want to get on with the football.
There may be stewards or police in the crowd who would notice who the people are, if there are chants when a black footballer gets the ball and the boos are coming from the same crowd, this will have been witnessed.