How I feel about this is that he owns the platform; he can allow or disallow any speech he wants. If he wants to ban all posts with the word “triskaidekaphobia” just because he feels like it, he can. No First Amendment implications whatsoever.
1A, just like 2A through 10A, restricts government.
The risk, as I understand it, is that social media platforms are protected against liability for what people post on them because they don’t exercise any “editorial function;” they’re supposedly just a platform where people post their own garbage. As I see it, once you start disallowing certain content, that protection should go away. “Oh, you removed all posts with ‘triskaidekaphobia,’ but you left up this post directly inciting violence that got somebody killed. Hear that whistling? It’s the sound of incoming lawsuits…”