He said this in 2016:

Look, having nuclear — my uncle was a great professor and scientist and engineer, Dr. John Trump at MIT; good genes, very good genes, OK, very smart, the Wharton School of Finance, very good, very smart — you know, if you're a conservative Republican, if I were a liberal, if, like, OK, if I ran as a liberal Democrat, they would say I'm one of the smartest people anywhere in the world — it's true! — but when you're a conservative Republican they try — oh, do they do a number — that's why I always start off: Went to Wharton, was a good student, went there, went there, did this, built a fortune — you know I have to give my like credentials all the time, because we're a little disadvantaged — but you look at the nuclear deal, the thing that really bothers me — it would have been so easy, and it's not as important as these lives are — nuclear is so powerful; my uncle explained that to me many, many years ago, the power and that was 35 years ago; he would explain the power of what's going to happen and he was right, who would have thought? — but when you look at what's going on with the four prisoners — now it used to be three, now it's four — but when it was three and even now, I would have said it's all in the messenger; fellas, and it is fellas because, you know, they don't, they haven't figured that the women are smarter right now than the men, so, you know, it's gonna take them about another 150 years — but the Persians are great negotiators, the Iranians are great negotiators, so, and they, they just killed, they just killed us, this is horrible.

He was still elected.

No, it's that they hear someone they understand. "He says it like it is," is white people speak for "his words aren't confusing me." Bc really all Don ever does is tell a story like a trailer park kid. It speaks to their biases and emotions and that's all they care about

I doubt most of them listen to him at all. They listen to Fox or Newsmax or whatever social media, who only cherrypick the fragments that sound vaguely coherent, and conversely only played the pieces of Biden's speeches that were incoherent. They either don't realize or don't care that they're not looking at the whole picture.

I expect that they are hearing but not listening

I actually laughed the first time I heard a Trump fan claim Biden was incoherent. I honestly thought they were kidding.

Nope, dead serious. "Have you ever heard Trump talk?" Silence.