I've never heard anything like that, sorry. It's true that Lincoln wasn't the GOP's obvious or first choice going into 1860, but a lot of Republicans (including Lincoln) are old line Whigs who fundamentally don't believe class conflict is good or desirable. Lincoln was the compromise second choice because the obvious frontrunner (William Henry Seward) was seen (unfairly, really) as too extreme to carry the free states bordering the South and had made a lot of enemies in his decades in politics. Lincoln was a relative unknown with few enemies who had gotten some good press for his debates with Douglas and far fewer enemies. He also got on much better with the farthest South sort of Republicans like the Blairs in Missouri and Maryland, which were understood by the party as likely spots for future expansion.

It is true that Marx, personally, supported emancipation and that might be where this is coming from. To the best of my knowledge, which isn't much on this front, there just aren't any Marxist organizations of influence in the US at the time. The closest thing might be disappointed socialists (usually German) who fled to the US after 1848. Their perceived radicalism and foreignness are hobbyhorses that slavery denialists have been tilting at for ages.

Can you post the source? I cannot answer this question, but posting the source will help whoever might take a stab at it.