I don't think it is a weak argument at all. Sure Reddit is a private organization but being so makes it a softer target for censorship advocates. They can try

I don't think it is a weak argument at all. Sure Reddit is a private organization but being so makes it a softer target for censorship advocates. They can try different techniques and arguments in that sandbox and get an amplified, but not dissimilar, return on their actions. It can also serve as a morale booster for those groups. As they hone their message and techniques against a large, democratic, and heterogeneous population, they can use those same tactics in society at large to provoke the same change in more inertial government.