Actually, the main point of the court ruling is not Marx himself, but the court's interpretation that the theories the plaintiff was discussing could lead to the "violent overthrow of the state and economy". That would indeed be a valid reason for declaring any such organisation unconstitutional and thus illegal.

The validity of the interpretation itself, on the other hand, is questionable - as the newspaper highlights, other courts do have a different point of view on this topic, so it is by no means a mainstream opinion.