[dragonwriter]

> Would that mean that if two price-dumping companies collude, a third party cannot sue them -- because there is "competition"?

No, if they are actually colluding, that is an illegal combination in restraint of trade independent of whether that collusion involves dumping, and anti-trust law provides both private causes of action (for harmed competitors) and public causes of action (for the government) to address such collusion.