Science is the business of trying to identify and systematize patterns in how the world changes over time. Marxism does that, and I think the emphasis on "how the world changes over time" dovetails nicely with dialectical materialism itself. That said, lots of other things are also attempting to identify the same thing (to a greater or lesser degree of success), so I don't think it's an incredibly precise descriptor. "Good science or bad science?" is much more informative than "science or not-science?" The demarcation problem is just bullshit.