Communism is post-socialist because it is what is compelled by the resolution of socialist contradictions. It's useful to have a demarcation between wholly socialist countries like China, which necessarily have large numbers of contradictions that compel movement forward, with a future communist society that will be free of such contradictions like private property. Socialism isn't something you can "partially be," really, it's a descriptor for the principal aspect of a society, what is rising within it. In that sense, it's similar to what you're getting at in socialism as a movement.