The conduct that the Sherman Act prohibits is broader than what you are describing. But even if it were not, the thing of value that is allegedly being extorted

The conduct that the Sherman Act prohibits is broader than what you are describing. But even if it were not, the thing of value that is allegedly being extorted here is control over brand safety policies. The advertisers are accused by X of colluding with competitors to use their combined market power to deprive X of more revenue than if each advertiser made its decision independently. I don't necessarily agree with the lawsuit, but those are the legal issues at play.