I find these kind of messages distasteful. When people are focused on building, they need feedback from the market. That rejection/feedback helps, but any other kind of rejection - especially a VC or investor rejection - hurts. You could argue that founders should be used to rejection, still this is a needless rejection - without any specific feedback and from a group not their TG - not something people should spend their energy on. You apply, there is hope, then there is a rejection (given the acceptance rate highly likely), then you see the kind of companies that got in, and it becomes a distraction. That's human nature. You can correct something when a prospect says they do not want your product and understand the reasons, but how do you correct anything when a popular investor passes on and says "they could be wrong"? Yes, "keep building", which they were already doing with more enthusiasm and less distraction before the whole bandwagon of YC alums came along.

You could argue that "filling the application gives clarity to a founder". Again genius marketing. That comes at an opportunity cost of talking to their leads, their prospects. At any rate, it takes you away from what you originally set out to do. You get more clarity from talking to your target market than filling an application.

Another thing is "people got in applying on a whim". Yes, those are exceptions. But, any average founder understands that if the decision is likely to be determined by what they write in the application and their application video (and several factors outside of their control), they will optimize it to the max. The videos you see are always multiple takes. The applications are always pruned and reviewed and rewritten before submitting.

When your goal is getting into YC, all this is worth doing. When the goal is building and growing a product, such advice, no matter how well intended, hurts more than helps the founders. It's okay if YC picks 200 startups out of 15000 instead of 20000.