X, formerly Twitter, has faced growing outrage since Media Matters published the report on Thursday, which led IBM, Comcast and several other advertisers to pull ads from the platform in response.
On Saturday, Musk posted that X would file a "thermonuclear" lawsuit against Media Matters and others "who colluded in this fraudulent attack on our company."
Since Musk purchased Twitter for $44 billion in October 2022, a stream of advertisers have fled the platform, wary of some of Musk's controversial posts and layoffs of employees who worked to moderate content.
X Chief Executive Linda Yaccarino told employees in a note on Sunday that while some advertisers had paused their investments following the report's publication, the company had been clear about its efforts to fight antisemitism and discrimination.
In an interview with Reuters earlier on Monday, Media Matters President Angelo Carusone said the nonprofit's findings flew in the face of X's statements that it had introduced safety protections to prevent ads from appearing next to harmful content.
"If you search for white nationalist content, there are ads flourishing. The system they say exists is not operating as such," he said.
(Reporting by Sheila Dang in Dallas, Editing by Rosalba O'Brien)
By Sheila Dang