JD was immune to blades, or at least ones that attempted to slice, cut, or pierce him. What did affect him was blunt force which usually forced the wind from his lungs, or knocked him unconscious. Of course if enough blunt force was present it would break bones, and bruise organs; but it would have to be a lot of blunt force. What affected him the most was magical attacks, and by the way that Haimon attacked he didn’t seem to have the right combination to take JD on. Still didn’t mean the man didn’t try.
The sword might have pierced him and wrenched so tightly in his armor that he would be forced to change back to assess and inevitably mend the damage- he wasn’t able to heal in this form. Yet before they were able to discover if that much force behind a blade could inevitably harm him- Silver managed to destroy it. JD knew his girlfriend was strong, but he was unaware she was this strong. He paused, and went to lunge at Haimon again- hopefully enough to distract him so that Silver could sweep in and finish the job.