I'm not the person you asked but for me, the idea that leadership is more in demand than intelligence doesn't ring true (anecdotally, of course). In the course of my career, I've met more than enough capable leaders, but finding people who meet the bar for intelligence is much harder. At some level, this is probably due to the fact that you need far less leadership than you do intelligence: by definition, leaders are a fraction of the workforce, but intelligence is useful at pretty much every level in the org (for white-collar work).
I guess I'm lucky enough that I've been in companies that were generally quite well-run; I'm sure the demand for leaders would seem higher in dysfunctional companies where competence in leadership isn't routinely achieved.
I find there's a strong bias against even attempting to make that judgement. "In hindsight" is used to dismiss criticism: sure, you can say that in hindsight, but I had to make the decision at the time with the information I had. But very few outcomes are actually entirely surprising, we weigh information and risk and probabilities, and these can be improved. An example I remember with particular frustration was Hillary Clinton, repeatedly confirming her decision about the Iraq war given the information she knew at the time. There was lots of information available, she made particular choices about how to weigh the information and risk, and while within her decision framework at the time the choice may have seemed right, clearly her framework was wrong even given her own conclusions. (It's also hard because we expect leaders to always be right, and if they aren't then they shouldn't be leaders, which leaves no room for improvement.)
If a ruler of a country never steps down off the throne and walks through the marketplace to ask the people what they need, how else will he know? Information gathering is connected to humility and manners in my mind. In order to be well-informed you have to be able to interact with different kinds of people and that requires a lot of social maturity, which according to the researchers, is apparently a "non-cognitive" skill.
And yes, I agree. In most white-collar jobs, intelligence is assumed. It's the non-cognitive skills (let's call it emotional intelligence) that are sorely missing.