Duh. Silly title. Of course they wouldn't know it if they had dementia especially among certain nationalities that generally lack self-awareness. I recall reading somewhere that dementia is quite prevalent among older foreign residents in Thailand due to loneliness. They aren't alone because they have people around them but they're (emotionally) lonely. I don't know whether this finding was anecdotal or based on scientific research but going around places like Chiang Mai and Pattaya where there are a lot of older farang one gets the vibe that quite a few of them aren't "all there" (which may not necessarily be clinical dementia but some form of personality disorder or social dysfunctionality ).

https://www.newsweek.com/older-americans-have-dementia-without-knowing-experts-reveal-signs-to-watch-12414002

Unfortunately, people with dementia rarely understand their cognitive decline. This fact seems to be part of the syndrome. It makes caring for those with dementia particularly challenging. My own mother felt her family, family physician, neurologist, and others were in a "conspiracy" to diagnose her with dementia. Despite obvious, persistent problems with her cognition, including leaving the natural gas on in the house, with the house filled with flammable gas, calling the police to report her car stolen because she couldn't remember where she parked it, and so on, she never accepted her diagnosis, and refused assisted living until the last moments. And, of course, we all know about President Sundowner, who rants all night and sleeps all day. At least the Dementia Don has guaranteed assisted living, paid for by American taxpayers.