The author points out, accurately, that had the 20th century seen an equivalent degree of violence as the average tribal society studied, we would have had 2B killed in warfare, not the ~100M we did see.
This does not leave out the first half of the 20th, during which most of the killing occurred.
You are conflating volume with incidence. More people were killed by violence during the 20th than any preceding century, but then there were a great many more people around to be killed. The incidence of violence went down, especially after the first half of the century.
Even long-ago “routine” wars had massive death rates. A recent study found the English Civil War, not one that is thought of as having massive atrocities, for the most part, leading to a decline in the populations of England, Scotland and Ireland of between 15% and 35%.
We see the horrifying events of our own time and don’t realize how awful the “primitive” wars of the past were.
For example, the Nazis were unbelievably inefficient when it came to killing masses of people. Their most efficient camp, Auschwitz, could handle maximum around 20k per day.
The Mongols routinely murdered 100k+ in well under an hour. Just distribute a half dozen tied captives to each man in your army, and on the appropriate signal everybody chops heads, with which you build a pyramid. Far more efficient than hauling people all over Europe. What the author of this piece was pointing out that such routine casual violence wasn’t practical even for a Nazi-indoctrinated society, forcing their leadership to resort to the very inefficient alternatives they used.
As another example, every time a dynasty collapsed in China, roughly half the population died in the ensuing disturbances.
Correct. Even Cromwell's actions at Drogheda were well within the established "rules of war" at the time.