"Collusion" isn't generally a thing. Providers can lawfully cooperate for all sorts of reasons; the exceptions are the cases they can't, not the cases they can. For instance, providers could run private channels for banning users that spread CSAM (and I'd be surprised if they didn't).
And in doing so, they would be forming a centralized information cartel. And the opportunities for abuse of such systems is now more than apparent. They have effectively booted a Twitter competitor off the internet, working in concert. If that's not legal cause for antitrust action, then antitrust is no longer a thing.
Do you have an argument that would be persuasive to people who don't believe that coordinated efforts to eliminate CSAM constitute antitrust violations?
