I've never understood why people say this. He's obviously extremely clever. I have no idea whether he's a 'genius', but he very clearly has a significantly above average intellect. Hence he went to significantly above average universities, was offered significantly above average internships, then a very significantly above average doctorate programme, and then enjoyed a very, very significantly above average career. The idea of him a achieving that as an idiot, like some modern-day Forrest Gump, makes him sound vastly more impressive than if he just happens to have been born with a big brain!
And if I, random person on the internet, am not convincing enough, surely the chorus of people who know him, or have worked with him, or who have a strong understanding of what it is to be a tech CEO, who to a man say he is extraordinarily clever, should be believed? Why would they lie? Even people who hate him say that, like Bezos when Musk was attacking him. Or Bill Gates who he gave a big interview saying how unpleasant and dangerous he was, precisely because he was extraordinarily clever. In fact, I can think of many people who despise him, but can't off-hand think of anyone at all who's worked with him who's ever said he's not smart. A nasty bastard yes, but definitely smart. Only occasional posters on internet forums seem to doubt it, most of whom I'm going to guess haven't ever met him... or been invited to undertake a material science PhD at Stanford... or started even one multi-billion dollar company.
It's a real bug-bear of mine (as you may be able to tell!); why make him sound more exciting than he is? He's a very, very clever guy with a horrible personality, and almost certainly some very severe mental health issues. That makes him a lot more dangerous, and a lot less impressive, than if he were some dribbling idiot who bungled his way into being the richest person on earth, and into the White House.