Our ancestor's sleep might not have been optimal due to the fact we had other things to care about, like shelter. But that does not imply there is nothing to gain from feeling perfectly rested.
Same argument when someone says "when I was a kid we did X and we lived". Doesn't mean sleeping on the back dashboard of the car on the freeway is a good idea, just that you in particular did not die from doing so.
"It survived evolution" = "It won't kill you" = "It's good enough"
They're setting the bar a bit lower than optimum.