This didn't make sense to me when I read it. What do we mean by "capacity for leadership"? Why would sexual orientation or gender identity impact a human's "capacity for leadership"? More qualification is needed here.
I looked at the original study (Dugan et al), and to measure "capacity" the authors used a self-report questionnaire measuring eight aspects of socially responsible leadership, including self-awareness, commitment, collaboration, common purpose, handling disagreement civilly, citizenship, and adaptability to change. The idea seemed to be to determine whether social experiences associated with sexual orientation or gender identity affect people’s access to leadership development and the way in which they are perceived, not whether queerness itself impacts a person's innate ability to lead. This is a very important distinction that does not come through in the current article (Holden & Bruce).
Also, the original study also only looked at the difference between heterosexual individuals and those who were lesbian, gay and bisexual. So, the use of the word "queer" here is misleading.