If you look back at the source for the "Humble People Make the Best Leaders" source, it's a little less straight forward than that. It's not about humble leaders, it's mainly a study about how altruistic leaders make their employees feel:
https://hbr.org/2014/05/the-best-leaders-are-humble-leaders
And that supposedly helped in somewhat ambiguous fields: Uniqueness, Belongingess, Team Citizenship, and Innovation