No.
This is the most common mistake the anti-Musk crowd makes. They fail to differentiate between a person being intelligent and having good self-control. They are not the same thing.
Musk failing at Twitter doesn't make him stupid. Musk having poor impulse control does not make him stupid. What you see in the Musk detractors is their own idiocy and or poor impulse control on display, in the way they go after him by proclaiming that he's stupid or the equivalent, it shows off their inability to think deeply and to control their own emotional impulses (to lash out; a failure to think rationally, a failure to be able to dissect a topic dispassionately). It's rather interesting that Musk's poor impulse control lures the same behavior out of his detractors (which is exactly what Trump's wild behavior did to his detractors, they have been wildly rabid for seven years non-stop; Trump acted wild, they reacted wild).
Intelligence is no guarantee of self-control, discipline, focus, integrity, work ethic, et al.
Smart people can be cruel. Smart people can be very lazy. Smart people can be liars. Smart people can be x y z. And in exactly the same way, smart people can have poor impulse control, behavioral disorders of all sorts, or just be lazy and not develop their work/task focus capabilities throughout adulthood.
There is also an interesting difference between smart/intelligent and wise. Musk is very intelligent, he may not be particularly wise. Knowing, understanding and accepting your limits is generally an area of wisdom rather than smartness. Properly acting on your acquired understanding of your limits would be maturity / personal responsibility (integrity is typically a major factor in that mixture). I don't think Musk is a very mature person, and of course his tweets have put that on display for many years now.
"Smart" does not just mean "high intelligence", as you very well know. In common parlance people use "smart" to mean an ill-defined combination of intelligent, knowledgeable, careful, and wise. A smart person is one who can deal with complexity and make good decisions.
Deliberately misinterpreting what someone has said in such a way that it is obvious nonsense, then claiming what they said is obvious nonsense, is not a clever trick. All too often effective, but not clever.
I am not willing to conclude Musk is especially smart, for the record. SpaceX seems pretty competent, but Tesla, Twitter, Neuralink, and several failed projects are technically dreadful in numerous ways. What can be attributed to Musk's intelligence, what can be attributed to his behavior, and what can be attributed to the fact that any fool with a big enough bag of money can hire brilliant people? I do not know.
What I do know is that every time he speaks on a subject I understand his remarks are word salad.
He could have spent that money on practically anything else and benefitted humanity. People would have been awarding him Nobel prizes and fawning over him for the rest of his life.
Instead he bought twitter and let the fascists run riot.
What is that exactly? Seems like he is primarily good at having money, well at least until he set so much of it on fire with his Twitter acquisition. Otherwise, what is his achievement that isn't effectively just throwing money at a problem in a way that no one had previously done?
Steve Jobs could have veered all over the map. He was anointed with the title of genius by the public broadly (and Jobs may have indeed been a genius at what he was great at, narrowly speaking). He successfully rebuilt a globally iconic company and his leadership resulted in Apple producing extraordinary products that hundreds of millions of people have enjoyed. He was also one of the richest people in the world when he died and quite famous. Instead of veering widely, he had the self-control to stay on task, stay focused, and more or less had the discipline to not veer far outside his sphere of competency. The self-control to stay on task seems to have been something Jobs intentionally made himself get better at over the years. He recognized the more important thing was to do fewer things and do them well and he preached that to anyone that would listen. He recognized that our time limitations (and time-productivity limitations (you can only be highly productive for a fraction of a day)) are aggressive, so you're better off focusing heavily rather than sprawling. We're quite finite in our capabilities, at best. Even when Jobs was doing Pixar, he was very careful not to go too far away from focusing on Apple, and he didn't attempt to keep sprawling his focus even though he easily could have (this is a big separation for Jobs and Musk; maybe someone with high productivity potential can do an Apple + Pixar simultaneously, with good management; do more than that? I think Musk has potently found the point of imploding returns on sprawling focus).
Musk is at risk of producing a lot of duds as he sprawls his focus, and maybe he doesn't care if that happens, however there is a significant time-productivity cost to what he is doing (and a huge monetary cost). The Twitter time and $40 billion could have been spent on getting to Mars. $40 billion would probably get SpaceX to Mars. This is something that makes me understand why some people like to hurl invective at him. Guy spends two decades with a supposed overarching goal of chasing Mars, acquires a quarter of a trillion dollars, some of which he can liquidate to accelerate the mission, and then he does this. Talk about veering off. Jobs would have sold $20 billion of Tesla and pumped it into chasing Mars, if that were his primary goal.
Despite how much certain people hate Musk now (in the same way they hate Trump, and for similar reasons), he is brilliant at what he's good at. No matter how much his detractors wish to tear him down, and no matter how much he makes it easier for them, it's simply true that he is a very intelligent person and at least narrowly highly competent. Most of the HN crowd would instantly vaporize when confronted with what he has taken on over the past 10-15 years (being worth $200+ billion and globally hyper famous would melt nearly everybody on HN, and that's just one point). Trying to do things well outside your competency, when you're already stretched thin on your primary mission/s, is a very bad mistake of impulse control / focus. Your line: "Why is it assumed that this guy who has no background in this industry knows what he's doing?" pretty well nails it.