Mises on Marx as a "proletariat"

From the Ludwig Von Mises Institute's Facebook page:

Of course, the Marxians consider a doctrine vicious if its author's background is not proletarian. But who is proletarian? Doctor Marx, the manufacturer and "exploiter" Engels, and Lenin, the scion of the Russian gentry, were certainly not ...

of proletarian background. But Hitler and Mussolini were genuinely proletarians and spent their youth in poverty. The conflict of the Bolsheviks and the Mensheviks or that between Stalin and Trotsky cannot be presented as class conflicts. They were conflicts between various sects of fanatics who called one another traitors.

--Ludwig von Mises. Human Action: A Treatise on Economics