But there's around 10 billion people on the planet, you can always find some unusual person to make a point.

In this case, though, she might agree with me, I'd say -- in that she spends part of her time reading and constructing things, from the article you linked: "reading and construction".

Modern tech? I think there's lots of cool not-modern tech, ancient tech.

Edit: She's an amazingly cool person! Thanks for linking the article, I might have a closer look at her & her life (now done. I wonder how many books she has to read, and after how long she's forgotten them so she can re-read)

Edit2: Sorry if my first version of this post sounded a bit grumpy. Now edited

My point is, our intelligence requires some activation. It is context-specific in the sense, that pure brain cannot develop it alone, in isolation or in a low tech culture. Configuration of our brain takes time and education, so modern human in prehistoric context may not reach the same IQ as if this person would have studied in one of the best schools on this planet. Agafia is modern human being, but she never had a chance to learn all the things that others had access to. When she met the civilization, she was already an adult person and her ability and desire to learn more was limited. She ran away from civilization back to her hut in taiga, and never wanted to live another life. She does not read a lot and, if you watch the documentary about her, she‘s reading Bible or books for children, so it’s not the same being fond of reading as for someone with university degree. She is unusual, yet she is normal, and she is a good illustration of what would happen if modern human had to become a hunter-gatherer, losing almost all cultural baggage except faith.

Do you have evidence for statisticly significant increases in IQ that persist into adulthood as a result of education?

Success in education is correlated with high IQ. Much the same way high IQ is correlated with many forms of success.

Education does actually raise IQ.

https://pubmed.ncbi.nlm.nih.gov/29911926/

…Whatever that means. It's meaningless to say fluid intelligence can't be improved with education, that's a tautology.

... we did not identify any studies that tested whether these effects persisted into adulthood. These estimates should thus be regarded with caution.

This is fascinating, and now I've fallen down "See Also" Wikipedia rabbit hole. I'm glad you shared this.