No, the censorship is not through the “proxy of corporations”, it’s directly instituted by corporations. It’s simply company leadership deciding they don’t want noxious people in their private club anymore.
Freedom of speech is not freedom from consequences. If you start talking and everyone tells you to shut up, then tough shit, you live in a society that doesn't like what you’re saying. This has always been the case, and it continues to be the case.
For many years, marginalized groups of all stripes were denied the megaphone of popular media, but they fought and convinced people they were right and built popular acceptance. If Kiwifarms wants to get the private protection and reach afforded to broader society then they need to convince the public that its actually good to harass and stalk trans people. Good luck.