I think there are a mixture, genuinely racist people, who latch on to the idea that taking the knee is "Marxist" as it's an argument against the knee that has been imported from the American hard right, as they want any way to undermine the protest against racism.
there are also probably some deeply unintelligent people who aren't really racist, but have been persuaded that BLM is "Marxist" despite no evidence for it, they've just been swept along with it. I don't think there will be many of these though