You’re missing the point. If he has dementia then his Presidential performance would show it.
So he either performed well as a President with dementia.
Or he is developing a new, extremely aggressive case of dementia that has been building just for the past couple months.
Well there is one more option that makes more sense to me. He doesn’t have dementia, is still a good President, and just had a bad debate.
But maybe I really don’t understand aging. Guess we’ll know by whether he is drooling and shitting his pants in the next few weeks as the “aggressive dementia” either does or does not manifest.