Of course Marx "would have done better to remember his own conclusions following the revolutions of 1848: for working class independence" but I think the question is how much of a grip he had on the situation in the US.
Mass communication and media now let us see what is happening anywhere in the world instantly. Marx had the bourgeois press and letters from his supporters in the US, mainly German emigres in the North. How much did he know about the split in the Northern bourgeoisie between Lincoln and the Radical Republicans, the state of the American labour movement etc?
Trotsky discussing with his US supporters in Mexico in the late 30's asked them whether black people in the American South spoke English. They were from New York and the Midwest and said they thought so but weren't sure.