I think the key point is more about how YC operates. At least from afar, it seems that they stick to the model that worked for PG and company, which means they strongly favor having cofounders. PG has also said, repeatedly, that they do occasionally found a single founder, so they're not dogmatic about it, just heavily biased.

An important thing to keep in mind is that you don't want a "cargo cult co founder", where you are trying to sign someone up "because you're supposed to have one".