Apple already lost here when it attempted to prevent people from even linking to other payment platforms. That was "their platform" until it wasn't, thanks to a court.
Giving them exclusive control to police speech - legal speech - without an opt out that doesn't involve thousands of dollars of loss is also "part of their platform" today.
I actually disagree with the ICEBlock app.. but that's for the courts to decide if it's legal, not a private company.