Dunne gave a perception of McDermott to the public that was written in a way to show his own perception of McDermott. That is some of the issue. He has done this every year since he hasn't been given media credentials for the last 3 years I believe.
So when he went to his 25 anonymous sources and said "I wanted to write a piece on McDermott", all he had to do was dangle the bait in front of the right person and wait for a bite.
As soon as something came out, he had a thread to pull on with the other sources as well and just kept chasing it.
Also, in regard to the sources perception, it's not that hard to get a negative take from some that is no longer with the team/company/organization in any field of work about their previous HC/boss/company.
Dunne did the same thing with Rodgers and McCarthy back in 2019 when he still worked at BR.