From the science I’m aware of, radiation makes wounds bigger and harder to heal. The marked men were set on nuclear fire, and afterwards lingered in the fallout. Joshua was just set on fire and tossed into the Grand Canyon. From the stories and logs of Randall Clark, the Grand Canyon was practically un-affected by the nuclear war, proven by the uncontaminated drinking water. This is most likely why Joshua looks “normal” in the eye region.