As been mentioned ad nauseam, this is not at all a random sample. The brains were taken from symptomatic people.
There are obvious limitations of the study that need to be further explored as well. What are the risk factors(alcohol consumption, age, race, weight, physical activity after retiring or not playing the sport, drug use etc.)? What would be incidence in a more random sample? It appears that linemen may be more susceptible compared to other positions(which makes sense), but what is the comparison of incidence rates holistically speaking?
I would guess, at least in reference to the risk factors question, that certain risk factors that exist for other neurodegenerative diseases may overlap to those in the case of CTE(excluding the obvious repeated blows and concussions). Similar neurodegenerative markers are present in CTE patients(pTau, amyloid beta).
To me, it seems like the link to repeated head blows is a major problem for the NFL, and perhaps other leagues if there such a significant relationship. Football as a whole may need to consider restructuring itself in the future, otherwise kids are going to avoid football except for particular sets of people(a la boxing).