Concussions don't apply equally to all players. Seen quarterbacks have their head bounce off the turf, no problem for some, but some get a concussion.
I read a theory a few years ago as to why that happens. It was that what causes a concussion is the brain slapping against the inside of the skull. If the person's skull on the inside is fairly smooth, you get a harder slap, sort of like slapping your hand flat on the surface of the water. Makes a big splash and can make your hand sting. However, if the skull inside is more uneven, part of the brain hits first, then the rest, which reduces the impact. Like curving your hand before hitting the water so that your fingers hit first, not the palm.
Or a tree with lots of small limbs falling on a roof and slowing the impact of the trunk.
Roger retired due to concussions, maybe the inside of his skull was more smooth than say, Meredith's, who got hit just as and probably harder and more often, but didn't (as far as we know) have concussion issues.
In any case, bet Jones won't be lowering his head or taking on linebackers as much anymore...