Karl Marx is not only the most influential economist of all time; he is many orders of magnitude more influential than Keynes. Only an American could be clueless enough to think this is up for discussion.
They're not wrong though. Marx is so influential even the people who abhor him have adopted his terminology (where do you think the word "capitalism" comes from?). Meanwhile, Keynes' ideas are pretty much forgotten, only coming up in fringe conversations about CBDCs.
I'm not an American. I'm also quite a fan of Marx (or, at least his analysis of the inherent flaws on capitalism). But Keynesian economics essentially runs the world. Marxism... does not.