If the cause is cultural, then it's not inherent and is tractable to change through education and is not an obstacle to rehabilitation. It's just a behaviour, not an innate trait.
Even if you take the rather extreme assumption of purely environmental factors, your conclusion does not hold. "Environmental" does not inherently mean "changeable" on already affected individuals. For instance developmental malnutrition is a purely environmental phenomena that leads to stunted growth, reduced IQ, and various other issues. It's completely environmental and completely irreversible upon already affected individuals.
And even if some revolutionary discovery was made to help treat stunted IQs, prisons are probably not the place one would want to start. The IQ gap holds between populations of all classes and socioeconomic condition. You open yourself to missing the true effectiveness of such a monumental discovery due to confounding biases within the prison population. The possibility, if not probability, of coercion upon experimental "treatment" towards prisoners is also probably something we ought probably prefer to avoid.
Low IQ doesn’t mean incapable of changing behaviour. You keep assuming this link but I don’t see any reason to assume low IQ prisoners are incapable of rehabilitation.
