I wouldn't call this a "churn announcement" - it's the biggest YC news in years. I guess I can think of a few different answers to your question. One is that since it's significant new information, it's on topic (job ads are the opposite of that). Another is that there's a special relationship between YC and the HN community, and it's good for the community to have space to discuss that. A third is that there is definitely no way that the community would support us trying to close such a topic to comments.

Also, job ads get placed on the front page by software. This submission got on the front page by upvotes like any other.