Karl Marx was a minor classical economist in the line of Smith to Ricardo to John Stuart Mill. He naturally favored free trade which was one of the earliest classical ideas shown by David Ricardo. [Some what like one might argue Keynes was not a Keynesian, in some sense Marx was not a Marxist.]
Marx was of course wrong on capital and many other of his ideas. Economics at that time was trying to understand value theory and Marx’s idea was one of several dead ends or incomplete answers. None of Marx’s mistakes transfer to David Ricardo from whose writings Marx learned that free trade was wealth and welfare enhancing.
Isn’t it true too that many American citizens believe in the principles of Marxism even if they reject the label “Marxist”? For instance, paying one’s “fair share” in taxes, “progressive” taxation, taxing “big” oil, taking care of grandma unless she gets TOO expensive, etc.