إِلَّا جِلْدَ الْخِنْزِيرِ وَالْآدَمِيِّ
The commentary distinguishes the reasons:
• Pig skin: tanning does not purify it because the pig is treated as inherently impure.
• Human skin: the legal analysis may give a purity ruling through tanning, but using human skin is prohibited because of human dignity and sanctity.
They should not be taught as physically impure for the same reason.