While they talk about egalitarianism and resisting domineering members, and various organizational structures, they don't establish that people followed "correct" individuals or that violence increases with increased population growth/agriculture/weapons.
I think I'm safe to say that there is plenty of evidence for violence that predates agriculture and modern society, even if agriculture and nation-states increased the scale and visibility of such violence overall.
I think replacing "nasty, brutish, and short" with its opposite "noble savage" ideal is probably just as mistaken of a projection from literature. Human nature doesn't seem to change much throughout history, tools might just make that violence more efficient and more visible.
I don't think your thesis is fully wrong, but I don't think it is fully right either, which was my point in pushing back against the earlier statements you made.