"Statistically your chances of succeeding are higher as a co-founder" is true in the sense that accelerators like YC have this as a selection criterion.
I'd be curious to see if this is true without the selection effect.
It’s difficult to know. Knowledge of such a study can get internalized into a founders psyche and become self fulfilling. And they are the type to do so (both seeking out this kind of report and being impacted by it).