I don't necessarily agree with the book, but the book does not argue about the superiority of an unfettered democracy - quite far from it.
At the same time I must say, I personally don't use this type of social media anyway because I think there is a conflict of interest. I use Mastodon, Pixelfed, Pleroma, etc. No company gets to regulate my social activity on these.
I don't think that this is censorship though, as it is not the government deplatforming these people and groups. It was clear from the start that private companies could do that and that it is well within their rights, the problem is that we rely on private companies for social media when we really don't have to.
I don't mean to be rude but that's not the best ideal: they had Socrates killed for "corrupting the youth" among many many other things nobody would consider democratic. Plato's Republic is also not very democratic to say the least.
Neither was ever about everyone having right to join debate. The "prevent your opponent from speaking" was routine aspect of their systems.
Donald Trump is now one of the most dangerous terrorists on Earth. No private enterprise SHOULD have any dealings with him. They especially shouldn't be hosting his violent and dangerous messaging.