Are you familiar with modern intelligence research? IQ test is reliable (get the same score when you test multiple times) and valid (correlate with things we actually care about, not just being good at solving IQ test problems).
"The scientific evidence is clear: IQ tests are extraordinarily useful. IQ scores are related to a huge variety of important life outcomes like educational success, income, and even life expectancy, and biological studies have shown they are genetically influenced and linked to measures of the brain. Studies of intelligence and IQ are regularly published in the world's top scientific journals."
My mum, after getting her PhD (and having taught herself through A-levels because her Yorkshire school stopped at O-levels, and got into Oxford) decided to take her first-ever IQ test. She rated mentally subnormal.
Astonished, she thought about what it was trying to test and took another test: now genius.
Up until I was eight they thought I might be deficient. So they poked and prodded and eventually some people came to school and took me to the library and gave me an IQ test. Then they came back and gave me another one. And I believe another one after that, by which point I was a little disgruntled. Didn't I already take one of these? Did you lose my answers?
Turns out that the test can be tuned to a range. If you take the wrong one it loses some accuracy at the low and high end of the range, so they had to retest me. They had assumed I might be dim and gave me the wrong one. The real problem was that I was so insufferably bored all the time that I wouldn't engage. But I liked puzzles and the test had a bunch I'd never seen before, so by the last one I was taking it in the spirit it was given.
My guess is in two parts. Either your mother took the wrong one(s), or took a bogus one, or you're thinking of elements of a successful adult that the test can't measure, like common sense and social graces.
In any group of peers at that level there are bound to be people who can certainly do the work, possibly better than you, but who are painfully, even cringingly, bad at certain other life skills. It gets a little uncomfortable to acknowledge these people as geniuses. And there are people who seem not to be all that, but on occasion surprise the hell out of you by coming through in a clutch.
I'm not sure genius is correlated with income, life expectancy, and even educational success, though. Never mind poor, miserable geniuses like Dostoevsky; even Alan Turing was considered a rather mediocre student (and was also pretty miserable and quite unsuccessful by many measures). I'm pretty sure there is some statistical correlation between IQ and genius, but the correlation is certainly not absolute. In other words, genius and very high IQ are certainly not the same thing.
Summary: "The quasiexperimental results suggest that the reform increased the average IQ score for Norwegian men by a statistically significant 0.6 IQ points."
This paper shows two things, although only one is surprising enough for publication. The surprising part is that "schooling in adolescence raises IQ scores", which is the title of the paper. Another is that IQ is so reliable that increase of 0.6 IQ points is statistically significant.
Intelligence: All That Matters by Stuart Ritchie is a good summary of modern intelligence research. https://www.amazon.com/Intelligence-That-Matters-Stuart-Ritc... To quote:
"The scientific evidence is clear: IQ tests are extraordinarily useful. IQ scores are related to a huge variety of important life outcomes like educational success, income, and even life expectancy, and biological studies have shown they are genetically influenced and linked to measures of the brain. Studies of intelligence and IQ are regularly published in the world's top scientific journals."