Thats wrong, which you'd know if you actually read it.
I have to disagree with you, all his arguments held back then, there was just less data. He also concedes that the statistical modeling relies on violence afflicting a proportion of the population, rather than absolute numbers - all well and good but an important distinction. Also your point is entirely fallacious anyway in the context of this thread. His book highlights a trend which cannot be disentangled from the historical context which gave rise to it.
I don't understand what you mean by "all his arguments held back then" when a significant portion of the book explicitly shows and discusses data regarding the number of wars fought during 1800 and before, compared to 1900 and early 2000. And the fact that another significant portion of the book deals with data about how much more likely a person is to die from violence in general during that period compared to the more recent period. I'm still convinced you have not read the book, and I suspect that is why your counterargument to it resembles a strawman.
All the trends are longer term, the theory about lessening violence first emerged in the 1700s[1]. WW2 doesn't change Pinker's conclusions, by his own admission. So, it is reasonable to conclude this book could have been written in 1930, and would be no less correct than it is now.