I find it pretty interesting that Martin does not mention the kind of community member-driven up/downvote mechanism found on this site (and elsewhere) as an example of decentralised content moderation. Edit: now I see Slashdot and Reddit mentioned at the end in the updates section (I don't remember seeing them on my first read, but that might just be me).
FTA: "even though such filtering saves you from having to see things you don’t like, it doesn’t stop the objectionable content from existing". He doesn't want upvoting/downvoting, he wants complete eradication (of whatever the majority happens to objects to right now).
I don't see that Martin Kleppmann is using 'democracy' to mean 'majoritarianism' here. He makes considered points about how to form and implement policies against harmful content, and appears to talk about agreement by consensus.
Democracy and majoritarianism are (in general) quite different things. This might be more apparent in European democracies.