Socialist market economies are a type of socialist economy, particularly in the primary stage. Socialism isn't a unique mode of production determined by purity, but just like the others, by its principle aspect. What might help paint a better picture for you is looking at Cheng Enfu's diagram of the stages of socialism:

China presently is in the primary stage, but is at this point well along to the next stage, the intermediate. These aren't hard lines or jumps, but gradually worked through and towards.