> 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.

Basically, yes.

Anti monopoly laws are meant to protect consumers, NOT other businesses.

It is very hard for a company to get in legal trouble for price dumping, because as it turns out, price dumping in many cases is freaking awesome for consumers.

Collusion's another thing all together, and not the thing you're replying to. Complicated, as always, but in short the scenario you describe might very well be illegal.