Marx was a bullshit artist and fraud. Engels was a lair and Marx's publisher and fellow con artist, and a better writer than Marx; don't know why he kept Marx around in the first place, since he didn't need him and had to fake the guy's math for him anyway.
Maybe, I have little to go on. But it seems a little harsh. Surely Marx should be credited with at least recognizing the corruption of previous feudalism/aristocracy, and how the industrial revolution was forcing cottage industries out of business. His promoted solutions now seem vague and simplistic, but that has the advantages of hindsight. And Marx was still ahead of the pack.
But you could be right that he merely was taking credit. I don't see much of a motivation for Marx or Engels to be corrupt because it is not like they had anything to gain by promoting turning the system upside down. And they essentially were right. Just that we found unions and government regulations to work instead of the more drastic options.
However, we have not yet faced the extreme realities of the predictions of Malthus, since we used fossil fuels to put that off. And when fossil fuels are used up, we may once again be facing the predictions of Malthus? Then perhaps more drastic solutions may be the ONLY options?