Whilst we don’t understand the correlation between genetics and intelligence on a sufficient level there aren’t that many genetic differences in fact you can’t necessarily distinguish the age of the sample that easily either unless you are looking at very specific groups.

And as far as modern hunter gatherers go we know that they aren’t less intelligent than urban humans.

We have sufficient evidence of advanced tool making and culture spanning back far enough through human history to indicate that intelligence isn't a modern fluke, heck copper mining and smelting dates back to at least 9000 BCE, the pyramids in Giza date back to 2600 BCE which means that we are closer to Caesar to today than Caesar was to the pyramids.

While I’m open to discussion that over 350-400K years some evolutionary steps happened as far as our intelligence goes. But I am more than comfortable with stating that plucking a human from 10K BCE and exposing them to modern education would yield an average human today.