I have no problem with such claims. It comes down to who says they have the sources and how often they have been right in the past. When Rudy Giuliani says “I have evidence but I can’t show it to you” you can be certain he is lying. When Jeffrey Goldberg says he was texted war plans you can believe him. In this case you can click on the author’s name and review reports from the past and see how those played out.