The legal test for duty of care and negligence is not whether the governing body had actual knowledge of the risks, it’s whether they ‘ought to have known’. They call this ‘reasonable foreseeability’.
It will come down to the period that the players were playing and what medical evidence was available at the time. Then what systems the governing body at the time had to ensure it knew about the risks and what it did in response (and whether the response was adequate to address the risk).
CTE was first discovered in 2002. I’m not a doctor. Maybe there was other types of known and enduring problems associated with head injuries prior to the discovery of CTE. That might limit damages for CTE prior to that time but leave the door open for other (probably lesser) claims.
It will be interesting to see how this unfolds and to see what changes to the HIA protocols are made in light of this claim.