It wasn't "one researcher", it was a group from the Boston University School of Medicine Center for the Study of Traumatic Encephalopathy. And there's no "review of Gehrig's medical records" since the Mayo Clinic sealed them. And he was cremated so there's no way to confirm it now.
https://www.pbs.org/newshour/health/...ehrigs-disease
https://www.juliestamm.com/blog/brai...neuron-disease