I haven't yet found a story that explains how it would be illegal for advertisers to decide that advertising next to neo-nazi content is not aligned with their brand. But because A LOT of companies decided exactly that, including an "advertisers alliance", it's apparently an illegal boycott that violates anti-trust laws. According to Muskrat anyway.
X’s lawsuit contends that advertisers in the past had to individually strike deals with social media companies to set boundaries around what types of content they would sponsor. Through GARM, advertisers have been able to aggregate their power, establish industry standards for content moderation, and enforce them. In X’s view, GARM now has too much say over the content social media platforms may allow.
To clarify, I'm not an antitrust expert, and I am a noted hater of Elon Musk.
But setting that aside, this is 1) ridiculous because it isn't an organized boycott but even if it were not spending advertising dollars is, like, not price fixing or collusion, 2) also championed by noted POS Jim Jordan, 3) part of a pattern and practice of Musk to just up and sue people when he's pissed, and 4) many of his other far fetched lawsuits have been tossed.