This is called "confabulation." It's a classic sign of dementia.
Parts of his mind no longer work, so the brain fills it in with falsehoods and crazy fantasy. But the dementia patient actually believes it to be true.
He's absolutely nuts. It's why there's a 25th Amendment.
Trump claims he was basically lifted out of the rubble on 9/11: "Two fireman, big strong guys, they grabbed me under the arms -- 'got to get our of here!' -- and they literally lifted me up, and they started running with me."