I think the main issue here is not alow of NFL players are dying that young and donating their brains to be looked at. It is presumed to be advanced because they don't really get a ton of brains that young. I suspect many NFL players around that age would have similar levels of CTE.
The effect of repeated blows/ concussions on the brain is far worse than we know it to be. It is just hard to prove until they can figure out a way to study people that are still alive.