You misunderstood, all humans have the potential for that level of bone density. It is lifestyle determined. Basically as your muscles pull on your bone, particularly in childhood through early adulthood, your bone mass increases. The more physical labor you do the more your bone mass increases.

So prehistoric hunter gathers had higher bone densities on average than contemporary farmers. And more than most modern humans. Not because our bodies changed, but because our activities did.

Of course all of these averages are on distribution curves. The exercise bit was a joke since the only modern people with close to the same bone density would be athletes