So if I defame someone in this comment, you believe Y Combinator should be held liable?
Contrast that to the following: I post a comment on Hacker News critical of some big megacorp. Megacorp can't credibly threaten to sue YCombinator over that because they are legally protected from being sued over the actions of their users. Megacorp sues me instead. I now have the ability to defend myself in court against Megacorp should I choose to.
So how could your second case occur?
If we were to apply US law, specifically the infamous "Section 230", then that would depend on whether Y Combinator actively engages in moderation (aka editorializing) of comments.
If Y Combinator only engages in moderation of comments merely to satisfy other legal requirements (eg: remove illegal materials), then Y Combinator is a platform and cannot be held liable for comments.
If Y Combinator engages in moderation of comments beyond any requirements mandated by law (eg: remove some comments because the moderators don't like them), then Y Combinator is a publisher editorializing its content and can be held liable for comments.
A simplification is if Y Combinator has anything to do with a comment other than blindly disseminating it, then Y Combinator can be held liable for the comment. If not, whoever wrote the comment is liable.
And yes, if you think social media is getting away with moderating and editorializing its contents (eg: "this is disinformation" notices) while evading any and all liability under the guise of being a platform, you are absolutely right.
Just because many people wish Section 230 says that, it doesn't. The the contrary it gives fairly broad protection to moderation efforts and does not say that moderation amounts to becoming a publisher. (hence various people calling for changes to this, restricting platforms ability to moderate)