It is understandable that the working class experience the historical redundancy of capitalism as an injustice, Marx is saying, but the language of justice, being derived from the very system of alienation that is commodity exchange can never be a basis to supercede capitalism.
I am afraid I missed the point in the exchange when the 'two Andies' said Marx's moral compass is not justice, but human flourishing, so I don't know which Andies, or whether that is what they said. But human flourishing is right. Marx's moral point is historical relative (but not relativistic). The question of right and wrong is not independent of what is possible given the existing level of human development.
So Engels says at one point that even slavery is an advance on executing your captives (the practice of tribes whose technical level is insufficient to absorb the additional labour of captives without jeopardising the consumption fund). Engels is not endorsing slavery, he is saying that the slave society was an advance on the one that executed its captives. Similarly, the Diggers' primitive communism in the English revolution was an idealistic fantasy without any foundation in reality - that could only come when human technique had attained the level of superfluity on which socialism could be built.
Marx's moral measure is what is best for human development given the material conditions. In the seventeenth century, that would be capitalism, however blood-stained its birth. By the dawn of the twentieth, capitalism had demonstrated itself hostile to human development.