This area of flesh and skin that glowed had been enhanced by Black Mantis's inner force. The surface of the skin and the muscles beneath it had been hardened into rock by force. So when Arthur's fist smashed into it, it was as if he was hitting stone, not flesh.
Meanwhile, Black Mantis himself struck Arthur's body. Arthur didn't have the force to protect him. But his flesh was indeed very hard. They were so hard that they too might as well be stone.
Arthur's flesh was so hard that Black Mantis had to protect and harden his fist with force. If not, his hands would be crippled from striking stone so hard.
If Arthur were wearing armor, Black Mantis's fist would have shattered it the way he cracked the sword, deformed it with his fist, and/or bypassed the defenses of the armor entirely with force to harm his flesh underneath.
