إِنْ أَنْتُمْ إِلَّا بَشَرٌ مِثْلُنَا
You are only human beings like us.
Rejecters of the messengers used their humanity as an objection to their prophetic claims. The restriction has a relative polemical force: an acknowledged human attribute is treated as excluding messengership.
The response need not deny their humanity. It rejects the mistaken inference that human nature rules out revelation or a prophetic mission. A true description and a false conclusion from it must be kept separate.