I'd say to "an acceptable level of moderation" - not perfect. But I agree with that statement if the company is itself profiting from the moderation. I think there's a separate example to be considered when the company is a "dumb host" - but as soon as the company starts promoting certain content with a complex algorithm (smarter than "most recent", "has most updoots", or even "has most updoots with a decay function") instead of just being a place people park their content then I think they adopt that responsibility. I feel quite similarly about things like Instagram and Facebook - I'd give a pass to Reddit because it basically does nothing outside of hosting discussions (similar to this platform).

But yea, if you're a business that makes money promoting other people's content then you're responsible for that content. Similarly, I'd argue that a skywriting company that writes libel in the sky should be held responsible as an accessory to the client if they were aware the statements were libelous.