Far from definitive.
Part of the hypothesis is a long latency, time to develop.
People just died before symptoms became obvious.
Even today, only a small number show the symptoms compared to the 1/3 that have the gum disease.
Modern incidence of gingivitis is estimated to be nearly 50% of adults. This, in an era of dental medicine and oral hygiene.
The elderly would have all been institutionalized a century ago if this had any merit whatsoever. Inflammation has much stronger correlation to disease of the brain than poor oral health.
It’s BS. Your “definitive” is straw man; got bias?