Moreover, all reasons given by YC as to why you should have a cofounder are qualitative e.g. https://www.ycombinator.com/library/8h-how-to-find-the-right....
Would love to see the statistical evidence because if anything it seems to be the other way round and this is pretty much just reflective of YC’s personal preference as it were (and many VCs prefer otherwise). Personally I would only advise someone to take on a cofounder if they already know them well and need their skill set. There is so much risk in a poor fit that it far outweighs any theoretical benefit.
So it seems like there could be a lot of adverse selection in your data where good solo founders don’t bother applying, or good solo founders take on value destructive cofounders just to satisfy YC.
That’s then a vicious cycle where YC sees increasingly lower quality solo founders which reinforces their opinions and data about solo founder outcomes.