Working in psychometrics I’m in the somewhat conservative “g is simply shared variance of many tests measuring human abilities”-camp.

https://www.jstor.org/stable/1412107?origin=crossref&seq=1

I’m not subscribing to the notion that g should be controlled for environment, quite the contrary, but if you do, what is left is the part of g which is genetics.

EDIT: The bit of knowledge I have comes from being published in psychiatric epidemiology on the topic of cognitive impairment and substance use.

That's not the claim you were making earlier. (Although, strictly-speaking, it's still wrong: many factors other than genetics are involved in producing a newborn infant, famously epigenetics. If you classify these all as "genetic factors", then yes, the claim is tautologically true by way of redefining words.)

I'd be interested to see how you'd go about controlling for those other things: so far, I haven't seen anyone manage it.