I disagree that it's a fallacy, the issue is that platforms have an interest in claiming themselves to be a public square. But the platform is not the square THE INTERNET, which is to say the open World Wide Web, is the public square. Platforms are the individual bars and pubs and cafes that line the squares and boulevards.
Each cafe and pub has the right to set its own rules for entry and set its own vibe and culture, but when there is an oligopoly of bars who are all megalomaniacs wanting to own the whole thing and keep people locked in their establishments for as long as possible, that does not lend itself to having a healthy community or vibe in any one of them.
Let people self-segregate and affiliate with the communities they want to. The racist shitheads will find their spaces, but they only start to multiply when they're able to wheedle their fearmongering propaganda in front of people who wouldn't otherwise want anything to do with them.