IQ isn’t reliable and hasn’t been for a very long time, if ever. People get smart in areas, not overall. The brain is like a video game in that sense. You get skill points and get to spec them into certain areas. Some people spread them around and get middling knowledge in all areas but you won’t ever find someone who’s a genius universally. It’s just not a thing.
Then you’ve got something else that is extremely important to this discussion. Arrogance. The smarter you are the more arrogant you tend to be. Usually not on purpose. Mostly it’s a byproduct of being told you’re smart constantly or by everyone else being slower than you and you get frustrated. When you have that arrogance you don’t seem to be aware of what you don’t know.
Which leads me to a third thing. Actual smart people don’t assume they have knowledge. They know they don’t know a lot and are really hesitant. Conversely, people who aren’t that intelligent are usually convinced they know more than they do. Why? Because they’re not smart enough to see gaps in their knowledge. They’re looking at what’s in front of them but not the overall picture.
All of that leads to this dude just genuinely not being smart. He was a combination of lucky and knowledgeable in a particular field.