I largely share your thoughts. I honestly expected Biden to at least be prepared enough to counter the usual Trump tactics of making things up and using strong words to impress his base while deflecting blame or critical questions.
He can't because he's 2637361873 years old and can't think fast enough to have actual replies. The rules of the debate were no notes and no communication with staff so Biden had to rely on what little working brain cells he has left to form responses and recall history. I also wouldn't be the slightest bit surprised to learn both him and Trump have been using stimulants, but only Biden stopped talking them for the debate since there was so much talking about that beforehand.
It was an absolute travesty that there was no fact checking from the completely useless moderators.