In my understanding, he's worrying that it's everywhere, all the time, in the self-censor sense.
I don't think he's worried about self-censorship in itself. If he's worried about anything in this essay, I would say it's the possibility of losing the option he describes here:
"The trouble with keeping your thoughts secret, though, is that you lose the advantages of discussion. Talking about an idea leads to more ideas. So the optimal plan, if you can manage it, is to have a few trusted friends you can speak openly to. This is not just a way to develop ideas; it's also a good rule of thumb for choosing friends. The people you can say heretical things to without getting jumped on are also the most interesting to know."
MY big opinion that I'm violating right now: Never Post. Especially about "cancel culture".
Cheers