I was under the impression that this idea of cofounders being a huge statistical success factor was pretty much just a myth.

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.