See discussion here Entry for National Laws / FRANCE - #21 by RouxRC
It was stated as non open because in addition to the open licence, a specific decree applies to this specific dataset creating legal insecurities towards its use. Surprisingly, they removed reference to it in the dateset description though, but it still remains in the law: https://www.legifrance.gouv.fr/affichTexte.do?cidTexte=JORFTEXT000000413818 (article 4)