Djilas explains from firsthand experience how the CP went from being a revolutionary vanguard to the new ownership class in the societies that they created. They didn't own the factories, mines, and fields by law, but they became the defacto owners (i.e., enjoying the benefits of having control) of these productive assets. Perhaps the CP bigshots and their bureaucracy didn't own title to these assets but they certainly acted and benefited from being in control of them all the same. And given human nature, perhaps this is inevitable too. That was Djilas' point.
Stalin, Mao, and Pol Pot just to name a few, all tried to prevent this but were unable to halt the evolution of the CP and it's bureaucracy into acting as a new ownership class. Look at the nominally Communist states that still exist: The so-called "People's Republic of China" is really a State-Capitalist enterprise; Orwell's Animal Farm as state policy. And Cuba and North Korea are simply monarchies under a nominally communist party. Witness the way Castro turned the State over to his brother Raul, and Kim Il-Sung turned control of N Korea to his son Kim Jong-Il.
And for all his prescience Djilas spent years in Yugoslavian labor camps too. This critique is far more effective at exposing the fallacies and failures of Marxism-Leninism in practice than all the screeds written by the Cold Warriors back in the day.