Except the only people branding BLM as Marxist extremists are their opponents, whereas the Nazis were actually Nazis.
There are Marxist extremists associated with BLM, and while "disbanding the police" may not be textbook Marxism, it is a "loony leftist idea" and therefore interchangeable with Marxism in the public discourse.
Similarly, very few of the people arguing for making the US a white ethnostate are literally "National Socialists". Yet, we call them Nazis.