I don't agree with your definition of censorship. A government doesn't have to be involved to qualify. Moderation is a subset of censorship.
Twitter (or any social media) moderating people's posts isn't wrong, for instance. Nobody's rights are being infringed when they do this. You might disagree with their moderation policies, but that's a different thing altogether.
Legally speaking in the US, it's only censorship when the government is doing it.