Musk is suing under antitrust laws. I’m not sure they work that way. Antitrust laws prevent producers from consolidating and exploiting consumers. However, when it comes to advertising placements, the companies are consumers, not producers, and I don’t think a consumer advocacy group (which the World Federation of Advertisers functions as in this regard) is illegal.
I don’t know, I am not a lawyer, but it seems like Twitter is grasping at straws here.