Interesting. Although:
Way, way more people play rugby than american football world wide, so comparing the total number of injuries is bogus. There are some 7 millions adults playing rugby in leagues all over the world. They need to compare the rate of injury.
Rugby players don't wear any head protection to speak of.
"Head injuries" and concussions are two different things. I can see quite a few head injuries that are not concussions to people not wearing helmets, like cuts, broken noses, black eyes, etc.
The NFL has been chronically underreporting the rate of concussions, as the article states.