15 points over two decades is an entire standard deviation improvement for the whole population. One-on-one tutoring has been shown to make a two standard deviation difference. That's the difference between median intelligence and genius. If that isn't large, what is? https://en.wikipedia.org/wiki/Bloom%27s_2_sigma_problem
Your study is about the effect of an extra year of badly done education on people who drop out of school, which is worth 1 IQ point. That same study shows that the value of mediocre education on people who don't drop out of school is 5 IQ points.
That improvement might come from all sources. Health and nutrition improvements can have very high impact. Deworming, using safer water. Contribution of education to this 15 points might be just few points.
> Contribution of education to this 15 points might be just few points.
It certainly isn't genetics. You're not going to cause a population-wide one sigma increase within a single generation using any genetic mechanism.
> It's not about IQ, also, was it ever replicated?
Yes, this has been replicated repeatedly. As far as IQ, if you apply for a private school that uses IQ tests for admission, you will find that all of them explicitly forbid test coaching because it is well known that it works. Even a video will cause large gains. https://www.sciencedirect.com/science/article/abs/pii/S01602...
No. It can't be 100. IQ tests don't require literacy. They don't even require language. You could administer IQ test to analphabet who doesn't speak you language and if he cooperates he could get result faily close to their best capacity that could be achieved.
https://www.psychologytoday.com/us/blog/brainstorm/201806/ho...
1 point per year of education is the lower estimate.