From what I understand "solo founder == bad" is a pretty common view held by both YC and a number of other accelerators. If they all believe this then it becomes a self-fulfilling prophecy, and smart people will rapidly find another founder before they go through YC whether they needed one or not.

According to Paul Graham the key reason why YC thinks teams are better is phycological support. But you can get that support by joining a proper peer support system instead, e.g. https://growthclub.online. Disclaimer: I am the founder of GrowthClub.