That's a dodgy argument there.
Yeppers. That's why I couched it the way I did. OTOH, introversion/extroversion is often reported by parents in the behavior of their newborns, so a genetic link is probable. Whether such a gene(s) will ever be identified - who knows?
My expectation is there is a handful of genes that influence such things and nurture simply enhances their effect. But then, I was raised by a strong intro- and an ambi-vert. I might've got it either way. What I do know is all the effects people report caffeine having on them I don't experience. I can drink a Mountain Dew and then go to sleep a short time later, where the uncaffeinated - having had their fix - can't nap out.
Being that it's behavioral, it's likely a combination of genetic predisposition, environment, and upbringing. Along with possible epigenetic interaction(s) also being a possible factor.
Being that extrovertion-introvertion is a sliding gradient, a more complex explanation is likely at play.
BTW, caffeine is one of the various poisons humans have an absurdly high (comparatively speaking) tolerance for. Theobromine is still king in that corner though.
EDIT: to avoid double posting.
It means lethal in 50% of the studied population, and, usually with humans not even that because the dose is an extrapolated estimate, not least because getting funding to do a study where you feed a group of people poison until they die is extremely hard to get funded these days. Unless you’re selling it to them first, of course.
We could use career politicians, tell them it's for a heart medication or stimulant (not entirely a lie), and do a double blind study.
I'm speaking purely in hypotheticals while being facetious.