That's a good point! I also recall the announcement [0] and related HN discourse [1].
Reviewing it now, it looks to me like they declared editorial independence without going into much detail. I especially don't see anything about guaranteeing the preservation of things that may harm the YC or affiliated brands.
[0] http://blog.ycombinator.com/two-hn-announcements
[1] https://news.ycombinator.com/item?id=10298512