Well you apparently assume that letter is damning. In reality it’s part of the GOP’s early association with radical politics.
Horace Greeley’s New York Tribune was largest circulation newspaper in the country as well as the house organ for the Whig and GOP parties, and Karl Marx would be one of its main writers for over a decade.
Greeley’s managing editor for the Tribune was Charles Dana, who became buddies with Karl Marx when Dana was in Europe for the Tribune cheering on the 1848 revolutionaries. The same year that Marx published his pamphlet which we know as the Communist Manifesto. This wasn’t a secret to Dana, Greeley, or anyone else.
“Well”, you might object, “Lincoln wouldn’t have known about that!”. Except that Lincoln owned a ‘48er German language newspaper, the Illinois Staats-Anzeiger, based in Springfield Illinois.
After the 1848 Revolutions failed the revolutionaries scattered around the world and in America they played a big role in founding the GOP and in gaining Lincoln the 1860 nomination. The Illinois Staats-Anzeiger was part of that which is why Lincoln bought it.
https://drloihjournal.blogspot.com/2020/08/abraham-lincoln-and-german-journalist-henry-villard.html
You have just provided us with some more evidence of it.