From the article:
Over the past two months, The Athletic spoke to current and former Eagles staffers representing a cross-section of departments and viewpoints about the inner workings of the franchise. They were granted anonymity to be allowed to speak freely about sensitive topics, describing an environment characterized by second-guessing, paranoia and a lack of transparency.
Good reporting from trusted writers doesn't need named sources. The integrity of reporting means they have vetted the sources and they are legit. Especially if you want to get the real scoop on things, these types of anonymous sources are just as good as named sources. And, like any good reporting, Pederson, Lurie, and Roseman were all given a chance to respond and participate in the story. They all declined.
It's pretty unlikely all 3 writers of the article (2 current and 1 former beats) and the Athletic itself, would put their careers and reputations on the line by using bad, or made up, sources.