I think that you hit upon the crux of the problem, Trump doesn't know fact from fiction; truth from untruth.
By all objective accounts in the universe, windmills do not cause cancer. The U.S. does not have a jet that is literally invisible -- and George Washington did not capture the airports during the Revolutionary War. It isn't much of a stretch to falsely claim that he won the election in a landslide; he won Arizona, Michigan, etc. A president that can tell us that a hurricane is going where the meteorologists say it isn't going, can just as easily tell us that his opponent stole the election.
Don't you think it's a problem when we have someone who was president, and wants to be president again, who thinks anything that he wants to be true is true?
Your defense of Trump is that he didn't think these were lies, he actually believes what he said was true. That's at least as bad. He tells falsehoods and lots of people believe him and this can and will eventually lead to violence. Truth matters. A president that citizens and allies can trust his word matters too.
During the Cuban Missile Crisis, Kennedy met with Charles de Gaulle, the President of France, to let him know what our intelligence had discovered. Kennedy had the U2 photos blown up and spread over a conference table. Kennedy told de Gaulle, these photographs are of Soviet missiles in Cuba. It wasn't easy for an untrained eye to make out the missiles but de Gaulle's response was telling. He waved off Kennedy, saying, “The word of the President of the United States is good enough for me.” Can anyone imagine a world leader taking anything Trump says at face value?