It is somewhat ridiculous to blame Marxism on Horace Greeley. The American Heritage editor surely meant this as a joke.
It is true that John Kennedy, like Barack Obama, had a much too intimate relationship with the press.
The article is very long. There's much more at the link. Sort of silly to ask what might have been except as an exercise in how history hinges on many things. The author of the article wrote a biography of Greeley in 1950.
Here is info about him on Wikipedia.
He was a Democrat. LOL.