The average number of founders in successful startups doesn't mean much unless it is from a random sample of startups that have both succeeded and failed.
If you looked at a graph of the total dataset, what you'd see is that almost all startups with only one founder fail. So the reason the successful startups you list mostly have two founders may not be because two is the optimum number, but rather because two is the next smallest (and therefore most common) number after one. We can't tell without additional data.
What I find interesting, is that a comparatively large number of companies on that page had only one founder, even though the odds seem stacked against them. Maybe there are substantial advantages to being alone. For one, you avoid having to design by committee.
If you looked at a graph of the total dataset, what you'd see is that almost all startups with only one founder fail. So the reason the successful startups you list mostly have two founders may not be because two is the optimum number, but rather because two is the next smallest (and therefore most common) number after one. We can't tell without additional data.