I think, at its core, the reason a co-founder is suggested is a person's internal dialogue is often different than the dialogue they have with others. Probably different parts of the brain (at least in part) get used when we speak with others than when we speak to ourselves.

Anecdotal evidence: How often have you gotten stuck on a problem, but as soon as you start explaining it to someone else you immediately know the answer?

Maybe those who are successful as entrepreneurs without co-founders have learned to exercise both of these dialogues alone?