I at least have read Marx. The idea that the government is a tool of class warfare is distinctly Marxist, as he's the one that made an argument for why this is. This is why Marx advocates for the State, before it is dissolved, to be put into the control of the proletarian class, the do callee Dictatorship of the Proletariat (which isn't a dictatorship in the common sense of the word).
At least from what I read, I don't think that they were arguing that Marx wanted the State to be an instrument of violence, just that he observed that the state was an instrument of violence for the ruling class, and made a good argument for why it would always be unless class itself was abolished. I think that they were referring to this intepretation, which is also compatible with what you said.