> Which means: for the people with a high enough AFQT score, IQ is correlated with AFQT score. This is indeed interesting, but generalizing to the rest of the population is a methodological mistake.
No, because everything else we know about IQ, especially the existence of positive manifold, strongly suggest that it will also correlate at lower IQs too. You can’t just say “the study on X has not included Y, therefore we should have absolutely no expectations about any sort of relationship between X and Y”. That’s not how science works.
Alas, there have been other studies done, and it turns out that indeed, AFQT/ASVAB score and IQ correlation does in fact extend to areas on the left side of the distribution too, exactly as expected. To reiterate: this correlation exists across entire spectrum, not only among high IQ people. If anything, the correlation at high IQ is lower than expected, because of range restriction effect.
> Moreover, there is no reason to assume the AFQT score to depend only on genetic factors alone
Fortunately, nobody is claiming that, so you won’t have to put much effort to convince anyone otherwise.
> no direct one-to-one correspondence AFQT percentile scores and IQ scores can be stated.
It can, though. The problem is that the population of military test takers is not representative of population at large. For one thing, its average intelligence will probably be higher, due to relative scarcity of mentally disabled people. However, if you know the parameters of IQ distribution among military test takers, you can easily convert AFQT/ASVAB to IQ.