Hmm.. This was remarkably imprecise language on my part!
When I say virtually indistinguishable I mean in the context of what you'd said, that the most primitive people now alive are "modern" and display "modern" social norms. Since the social norms we're referring to are those related to breeding, what I am saying is that the most primitive people alive (the Sentinelese) are no more "modern" in the presumptive sense (e.g., post-Industrial) with regard to breeding norms than were the contemporaries of the Neanderthals.
There, that's more precise!
On second thought, considering the time scales involved, post-urban would be a better standard for "modern" than would be post-industrial.
But even by that standard the Sentinelese are hardly modern! They are pre-urban. And they aren't the only ones either. So, to wrap my part in this whole digression up, my conjectures have been extrapolated from the behaviors of non-urban subsistence tribes.