To be fair, you don’t know for sure he had it back then (or even now), either. Not unless you’re a certified mental health professional who has had enough sessions with him to make that call.
Apparently not, because I knew for sure he had it, and was right. In fact I have a 100% accurate track record in identifying people with dementia. So the idea that I would need to be a doctor and have many sessions with him to know for sure clearly isnt true since I have shown a near perfect accuracy in the past.
Dementia is easy to diagnose. What isnt easy to diagnose (and what i made no attempt to do) is what kind of dementia he has. In retrospec we can speculate even on that given the rapid onset (its likely lewy-body), but yea the specific form would need some tests. But to just say he has dementia, that is pretty trivial and not a hard call to make, like at all.