People do say those things interchangeably, but only at the cost of clarity. One is clearly correct and the other isn't, even if sometimes the folks who write dictionaries sometimes have to accommodate them so that confused readers can understand wtf is going on.

To understate is to make something sound less severe than it was. If you CAN'T understate the inhumanity of medieval warfare, then it simply wasn't inhumane at all. I know you think we're being pedantic here, but I genuinely had to read your title a couple times to realize what you'd meant. At first I sincerely thought you were saying that medieval warfare wasn't as bad as people think it was. You should change it if you care about this. "The article is doing well either way" seems to reveal a concern only with your own metrics, and not with the actual point you're supposedly making.