Talked with a friend today and casually mentioned the book, and she got furious and mentioned a bunch of problems with the book and the author. I advise people to look for criticism about this book before using it. The only specific thing I'll point out here is this:
"In 2019, a group of certified NVC trainers published a #MeToo statement honouring Marshall Rosenberg's legacy but also acknowledging he had slept with students at some times of his life."

LOL pretty much went the same path as you. Tried some other search engines (don't have a searxng one though). And it's only that IRC log. So, personally couldn't find any evidence. But also doesn't seem that important, since the concept appears to be the most important part.