That's one way to spin it. Another way to spin it, which is how it will be presented in court, is this "cartel" is tasked with advising their clients on how to direct their advertising budget to maximize their reach and protect their brand.
How about individuals? Is my not buying a Tesla in order to deny that revenue to the Musk world considered illegal?
Sort of. Musk has been attempting to weaponise litigation. The transparent aim of the suit is to bully departed advertisers to spend less on ads than they would on legal. The oversight being those are different departments, with the more-likely result being the manager who ran ads on X being fired while legal sutures the wound. (Even the suit being against the trade association defeats its own purpose.)
Ironic since Citizens United enables billionaires like Musk to use their corporate entities to engage in political speech. Musk is just a child who expects the world to bow down in his presence.