Were they correct? I guess that depends on which leftists you are asking about. The article jumps right into praise for Marxism, and only at the very end mentions other socialist models, all without defining or distinguishing any of them one from another. I often feel that we more-or-less left political advocates discuss these economic/political issues without any clearcut defining of our terms, such that we end up have to guess haphazardly what each other is saying and may use the same labels but mean somewhat different things.
I remain very leery of Marxism, per se, because while Marx was great at articulating real weaknesses and abuses of capitalism, he seemed supremely naive in the question of how societies were to get to a socialist state of being. ....Or maybe that was just how those who took up the mantle of communism chose to operationalize it with armed revolt and revolution.
Revolutions are not only dangerously unstable; history has shown us they lack an effective mechanism for preventing strongmen from seizing power and ushering in dictatorship. (Dictatorships also spring up in capitalist systems).
Capitalism and socialism share something in common that few want to talk about. That is that both are ideals, NOT pure systems in the real world. They each exist by matters of degree, not as absolutes. Societies, in actuality, wrestle with the debate over HOW socialist vs. HOW capitalist to be, NOT which one to be.
My personal preference is to try to emulate Norway, a compassionate eco-democratic socialist endeavor that nurtures a healthy social safety net of healthcare, educatipn, environmental responsibility, a truly inspiring rehabilitative penal system, government supported daycare, systematic respect for science, and on and on, even as it recognizes a place for free enterprise.

Saw that myself also

Capitalism must fail and it will fail first where it is most developed, here in Amerika. The failure will be ugly.

Yes they were..and are..thankfully..

Interesting analysis. Does make sense.

Good post. Scary, but good.