Well, if a human being has a soul, and something does not have a soul, then it is not a human being.
The Sacred Congregation document in question does not say something lacking a soul is not a human being. But CC 365 tells us a human being has a soul. So, putting 365 together with the document from the Sacred Congregation, we can see that a human being has a soul. Therefore, if ensoulment has not happened, that which does not have a soul is not a human being.
*CC365: ā365 The unity of soul and body is so profound that one has to consider the soul to be the āformā of the body:234 i.e., it is because of its spiritual soul that the body made of matter becomes a living, human body; spirit and matter, in man, are not two natures united, but rather their union forms a single nature.ā *