You'll have to take his word on this one. He had the evidence to back it up, but his dog ate it. And then the dog was drowned when his office flooded. And if the Italians claim he was never there, maybe he meant France. He can't recall.
Your comment prompted me to look on "Google" for "Giulio Douchet". Google had no hits for that exact name, but asked me if I meant "Giulio Douhet". Apparently Bellesiles (or the newspaper?) spelled the name wrong - how fitting! (Douchet sounds more French than Italian, as you imply).