Trump is just Trump. He is a narcissist, of course. I suppose you could call that an impairment, but that opens the door to diagnosis for anyone who seeks the office of president.
Trump's mistakes like the one pointed to in your first post are the types of things Biden was doing 40 years ago when he was just a moron (actually, milder than what "Uncle Joe" was doing decades ago, and frankly they are the types of things that anyone does when they speak for hours every day in front of audiences--something Harris doesn't do). That doesn't mean he is medically cognitively impaired due to something that often but not always happens to some people when they age. (I was using "dementia" as shorthand for "medically cognitively impaired due to something that often but not always happens to some people when they age," but Biden definitely has issues beyond mere old age.)