"Co-founders make you stronger" is a questionable generalization. Cofounders add a lot of things, both positive and negative, and as long as you and (each of) your cofounder(s) have vital skills/resources that are unique in the group, you now have multiple "single points of failure."

Yes, cofounders can contribute bandwidth, skills, connections, and all sorts of other positive things. They can also contribute friction, disagreement, and various other negative things. It's not a cut and dry benefit.

As a solo-founder, I've never run into a customer that cared how many people were involved in my company. Once you get to the point that things look professional, most just assume you aren't running the company out of your apartment. Maybe this matters more for sales-intensive B2B startups though.

Cofounders with good relationship > sole founder >> cofounders with poor relationship or other issues.

Co-founders make you stronger, co-founders make you weaker. There are plenty of examples of both. There are examples for every narrative. There are no absolutes in startup success.