I had to skim quite a few down the search results to find an article that described what it meant by suing for "illegal boycott" in more detail.

https://variety.com/2024/digital/news/elon-musk-x-sues-advertisers-garm-boycott-1236097110/

X’s lawsuit alleged that the advertisers’ “boycott” violated Section 1 the U.S.’s Sherman Act antitrust law, which broadly prohibits agreements among distinct actors that unreasonably restrain trade, “by withholding purchases of digital advertising from Twitter.”

“The conduct of Defendants and their co-conspirators alleged herein is per se illegal, or, in the alternative, illegal under the Rule of Reason or ‘quick look’ analytical framework,” the X lawsuit said. “There are no procompetitive effects of the group boycott, which was not reasonably related to, or reasonably necessary for, any procompetitive objectives of the GARM Brand Safety Standards.”

The “unlawful conduct” alleged by X is the subject of “an active investigation” by the House of Representatives’ Committee on the Judiciary, the lawsuit said. The committee’s interim report issued on July 10 concluded that, “The extent to which GARM has organized its trade association and coordinates actions that rob consumers of choices is likely illegal under the antitrust laws and threatens fundamental American freedoms. The information uncovered to date of WFA and GARM’s collusive conduct to demonetize disfavored content is alarming.”