You are right that he wrote about it, but it was in his book To Kill A Nation, and not Against Empire. Here is the direct quote from the book:
As the war dragged on and NATO officials saw press attention drifting toward the contrary story—namely that the bombing was killing civilians—“NATO stepped up its claims about Serb 'killing fields,'” notes the Wall Street Journal.2 Widely varying but horrendous figures from official sources went largely unchallenged by the media. Support for the bombings remained firm among Clinton supporters in Congress (including the one professed “socialist,” Bernard Sanders [Ind.-Vt.]), and among self-described humanitarian groups such as Human Rights Watch, Doctors Without Borders, and Concern Worldwide, along with “peace” groups, and various NGOs—many of whom seem to have convinced themselves that NATO was defending Kosovo from a holocaust.
Maybe it was Inventing Reality then? Fuck, I dunno, I know he talked about his friendship with Bernie for maybe a paragraph or two, not just that blurb.
This is really bothering me now, I'll find it soon!
Source that backs up your comment:
https://youtu.be/OLNQEHbusSA?si=_nP8pMiD5DXJ0XMm
I believe he also personally wrote about it in Against Empire. I know I read that story from his POV in one of his books.
You are right that he wrote about it, but it was in his book To Kill A Nation, and not Against Empire. Here is the direct quote from the book:
Just searched through my copy of both Against Empire and To Kill A Nation to make sure 
Maybe it was Inventing Reality then? Fuck, I dunno, I know he talked about his friendship with Bernie for maybe a paragraph or two, not just that blurb.
This is really bothering me now, I'll find it soon!