They took men and women who were both gay and straight and demonstrated a bias in expected leadership ability based on perceived sexual orientation. They are not comparing charisma, the voice selected where selected specifically to have a high likelyhood of being attributed as gay or straight.

Arguing that a difference in outcome from this is because CEOs all sound charismatic and that they are able to pick up attention of a crowd better when there is a demonstrated bias between the two when the difference involved is specifically whether a voice is perceived as gay or straight implies that gay speakers are less likely to have those characteristics. Which is exactly what the paper's conclusion is, a bias against people who are perceived as gay for leadership positions based on their voice.

    We investigated this issue in four studies (overall N = 276), conducted in Italian language, 
    in which heterosexual listeners were exposed to single-sentence voice samples of gay/lesbian 
    and heterosexual speakers. In all four studies, listeners were found to make gender-typical 
    inferences about traits and preferences of heterosexual speakers, but gender-atypical inferences 
    about those of gay or lesbian speakers. Behavioral intention measures showed that listeners 
    considered lesbian and gay speakers as less suitable for a leadership position, and male 
    (but not female) listeners took distance from gay speakers. Together, this research demonstrates 
    that having a gay/lesbian rather than heterosexual-sounding voice has tangible consequences for 
    stereotyping and discrimination.