That paper disagrees with the established view, but helpfully lays out what the established view is. Scanning papers on the topic it seems like there's mixed results war to war, and it depends what other things you try to control for - I think you could get to saying it's neutral but not that there should be a strong presumption that a war will suicide.

"the argument of "increased social cohesion" against a foreign enemy is not applicable to a civil war"

You'd get increased social cohesion among each of the ethnic groups which is in conflict with the other.

In any case this focus on the Sri Lankan civil war specifically isn't that interesting - I posted a review article above which includes controls and studies a wide range of cases across different countries, and concludes pesticide control has large effects.