Piketty himself is a social democrat who abjures the Marxist label. But as his title suggests, he is out to rehabilitate and recast one of Marx’s key ideas: that so-called “free markets,” by their nature, tend to enrich the owners of capital at the expense of people who own less of it.
On the right, being insufficiently worshipful of capitalism as they define it is now the same as being a Marxist. It doesn’t matter how you reach your conclusions, it doesn’t matter if you disagree with 90% of Marx. If you do not follow the right-wing party line on capital, it’s the same as being a devoted Marxist.