It's worth listening to the 30 minute explanation on their podcast feed if you want a legitimate explanation to that question. They really try to account for it

It's worth listening to the 30 minute explanation on their podcast feed if you want a legitimate explanation to that question. They really try to account for it. They relied on the gut-check from other terrorism experts, what was publicly available about Canada's investigation, and got a verification from a field commander in ISIS who said that he remembered the fraudster but the fraudster didn't serve directly under him like he had claimed. In retrospect, the hook of the story was so strong that they hadn't subjected it to the level of scrutiny of their other ambitious projects. He contrasted it with the Trump tax return story, where he had personally reviewed so many versions of that story that he had memorized the details. When they gave the same investigation to another investigative team who was familiar with the terror beat (with the benefit of hindsight), that team concluded that the story was a hoax.