I don't find the explanations in the article convincing and I think they omit just how much more horrific WWI was than anything that came before. Indeed, WWI started with the kind of positive perception you'd see in previous wars but its intensity, brutality, and scope put those perceptions to rest. New weapons systems like coordinated mass artillery and gas attacks are unprecedented in history and, from all accounts, are a horrific experience even if you survive. And if you do survive, you're likely to be sent back out to face another artillery barrage shortly.

In short, I think the analysis in the article has cause and effect backwards. We didn't suddenly realize that warfare was evil thus making PTSD more prevalent. Instead, the experience of a war that left so many of its participants mentally scarred changed the perception of war in society. PTSD led to changing attitudes towards war, not the other way around.