What I think the reference is to is

The “dangerous class”, [lumpenproletariat] the social scum, that passively rotting mass thrown off by the lowest layers of the old society, may, here and there, be swept into the movement by a proletarian revolution; its conditions of life, however, prepare it far more for the part of a bribed tool of reactionary intrigue.

from the chapter 1 of the 'festo, and even though it's harsh it does state that it's not an inevitability, but rather a more likely outcome.