The way I interpret the quote is that war always carries themes of heroism, sacrifice, adventure, etc. Ideas that always make it compelling. Even a film that portrays war in the most brutal unromanticized way still ends up glorifying it simply by engaging with these themes.

What you go for is a "damn I never want to go to war" reaction but what you get is "sucks those guys died, would have been different if i was there though" and that is the "glorification".

I think this is what Truffaut meant that it's a poor medium to convey conflict as the lens must glorify otherwise its not longer a movie, just footage.