I'll put it in a simpler way for you then.
The guy that "can't form sentences" still showed a far better grasp of the things a president needs to be able to do than Trump. What does that say for Donald Trump? He looked better losing to a guy who you said is too cognitively impaired to be president, but Trump still lost on policy, demeanor and grasp of the issues, just as he lost on the metric of what each accomplished in their terms in the office.
Trump lost to the guy you say shouldn't be president, ergo Trump is completely unable to be president, either.