At this point, it seems like YC is just optimizing on execution risk, and only care about those startups where market risk is eliminated (more or less). If someone has already invested in more than half the batch, YC's already has a pointer about a big well defined market (or expected to be), just needs a team which can execute well.
People used to apply to YC because they would be able to get support without a network. If a startup has to raise funds before getting into YC, I don't know, just does not sound right.
This may also be the reasoning behind accepting many startups on just basis of an idea(29%) provided they worked in big-tech or hot startups previously[1], and working on problems in Dev Tools and B2B Saas space (similar to problems they solved already, or seen). Market is defined, they are just looking for good executioners. (In dev tools and Startup related Saas tools, market is other companies of the batch on occasions.)
[1]: YC indexes heavily on anyone from an Ivy League college, or who has worked at FAANG, or unicorn startups like Uber. Going through their companies list, ex-FAANG/Ivy leaguers building a dev tools (or planning to) is the most repeated pattern.