In my point of view a man with a deep understanding of its time is congratulating another one for successfully hacking a vicious system. Don't let the 20th century marxists (or the cold war propaganda) ruins your perception of Marx's genius. Lincoln's importance is more obvious. Both men changed the world for better (IMO Marx is not to be blamed for any dictatorship atrocities). A Nelson Mandela quote resume pretty much for me both of their ideoligical legacies: "Money won't create success, the freedom to make it, will."

That said, I think its counterproducent for people who love to claim that are trying to change the world, underestimate the significance of history. And this letter is, at least for me, a unique, yet not well known, piece for understanding history.