It can mean they suffered serious health problems for 15 years until a particularly bad seizure or some such took them. TBI can cause long-term conditions like epilepsy, physical disability, organ dysfunction, sleep problems etc.

I was very struck reading about about a mass-shooting "survivor" dying 20 years later and it being considered a murder. It sounded wrong but when reading the account of the terrible health problems they suffered every day until their death, I was left in no doubt it was not just murder but a far crueler one than those that died on the day.

That makes sense. I naively assumed that if someone had survived ~15 years they would more or less be likely to live as long as anyone else, but on reflection there's no real reason to believe such a positive thing.