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A "Marxist" is someone who adheres to the philosophy of Karl Marx. It's as simple as that. And Hitchens hasn't been part of the left for a long time, so he certainly hasn't portrayed himself as a savior of it.
Finally, a person can be a humanist while criticizing the spiritual beliefs of others. Especially when those beliefs cause (and call for) a lot of injustice towards humans who don't share them.