From what I understand, the client says the cube is moving, however the server says it is not. So in the client you can move straight through the moving cube as the server has authority (since it says its not moving).
But what i found weird is that when i jump on the static cube (on the server), the client says that the other player is on moving cube (and moving with the cube) which is really weird. But when the server jumps off, he appears to have teleported (according the client)