there is absolutely nothing stopping a poor child in sierra leone from becoming the next einstein, outside of access to things that should be considered mandatory for human life.
According to research, a full education can add ~15 IQ points.
So someone from Sierra Leone who's average and receives a full education can expect to have around 77 IQ, which means severe issues with reading comprehension, math beyond simple arithmetic and following multi-step instructions.
i would like to know if that child was fed well, provided with shelter and given a good education -- would it still be, in your words, "not possible" for them to break even 100IQ?
Environmental and educational impacts on IQ are very minimal. Someone with an average Sierra Leone IQ is not becoming a genius after world-class education and the best environment possible. They'll be measurably improved, but not by 50 IQ points.