Not the guy you replied to but, yes, it often is more or less just "osmosis". That's how early children learn most of their basic skills to start off, and so long as they grow up in civilized society there are a lot of things which more or less come naturally to most children even if they grow up in shitty conditions. There are also obvious variances between people who grow up more or less in the same environment.
You could argue whether those differences are significant, or whether they constitute a reason to enforce political class distinctions - I don't believe that so-called "geniuses" are in fact anything special or far beyond the rest of the scum that is mankind, but rather just indicate that someone was well acclimated to the test - but there is definitely going to be a difference between someone in the 90 and someone in the 110 range. Whether that should be a life-or-death political distinction is a different question, and I think that would be absurd. I can also talk about whether the difference between 90 and 110 matters that much, and question the methodology of the test (much ink has been spilled on the latter, but curiously little on the former). It is curious why we make general intelligence such a life-and-death political question and wank over it so much, when it is obvious that even the brightest human being is pretty fucking stupid.
I have a developmental disability and all the shit that comes with it. That I even wound up with an average score was surprising because by that point I had checked out completely. When I was 5, I had the instinctive fear that if I failed the test, my life was over, so my mother essentially trained me to the test. I don't think it had anything to do with spectacular mental development, although the pattern of my development seems to repeat often enough in the stories I read from other high-functioning autist types. I knew damn well throughout my childhood that I had extreme deficits in functioning, so I wasn't some absolute genius, despite what some teachers told me (the same teachers who would immediately laugh behind my back about what a retard I was).
IQ must be seen as a static property because of how the emerging form of political economy ties "merit" to worth and the property one is allowed to own, the influence they are allowed to have in society. That, in my view, is why it is common to say someone "has" autism, or "has" a genius Autism Level. It's their ticket to societal worth and status, even though both of these things are essentially political labels. There isn't an actual "thing" you can look at in the brain or psyche and say, "yup, that's the autism" - it's literally just a checklist of observed behaviors and functional deficits, and anyone trying to work with the developmentally disabled isn't trying to "cure the autism" so much as look at each area of deficit and try to manage or account for those problems (or, more often, ensure that the autist remains in their designated political place, which is to live to be the target of humiliation). Autism Level is just a shorthand to describe a mess of presumed psychological and neurological factors that match up with general intelligence, that we don't quite know how to define exactly; but in practice, Autism Level sortition enforces the placement one has in "meritocracy" from an early age, and the system itself is loathe to move people from one part of the hierarchy to another even in the face of overwhelming evidence. When it was obvious that I wasn't actually a genius, the system could not fathom that I was anything other than a drooling retard. The thought that I could be just "average" was abominable to the operators of the schools, and they really, really don't like the thought that I could do things like drive a car, work a menial job, or be even in the slightest position to cause trouble. So I was routinely degraded, denied textbooks, and so on, as is the case with speds in general. The thought that, after all that I went though, I was just a relatively normal human being with some issues, was too much. I just HAD to be deemed a retard and degraded.