We all knew he was non compos mentis long before he even was elected (so to speak) President. Thus the basement campaign, the appearance at a car lot talking to a bunch of new Jeeps. And then, after the 2020 election, the constant vacations in Delaware, falling off a bike, "C'mon man" and "Oh, you know, the thing."
Who didn't know he wasn't mentally all there? The debate was just one in a long line of examples.