If it's valid to criticise a critic, it must also be valid to criticise a critic critic.
The merits of a given argument or action do not rest on "because daddy said so". There is and must be a daddy, but no daddy is above critique themselves even though their job is to moderate others.
IIRC, the downweight was correct in principle, because the subthread wasn't about the particular story, it was just a generic complaint about the general topic. We want those downweighted because, regardless of topic, they have a strong tendency to sit at the top of a thread, accumulating upvotes and choking out more specific and more interesting discussion.
It was a mistake to downweight it, though—not because the subthread was good, but because the topic was a YC startup. By far the most important rule of HN moderation is that we intervene less, not more, when YC or a YC startup is the topic (https://hn.algolia.com/?dateRange=all&page=0&prefix=false&qu...).
The user who did the downweighting didn't know that Fig was a YC startup, so it was an honest mistake. Certainly none of this has to do with trying to suppress criticism of YC or YC-funded companies. We know how to take our lumps and do so all the time. We also know that community good will is the only value that HN actually has, so doing anything to jeopardize that would be really dumb.