There's no mention in this post about controlling for extraversion. A narcissist is an extravert, almost by definition, and because of that the NPI test they li

There's no mention in this post about controlling for extraversion. A narcissist is an extravert, almost by definition, and because of that the NPI test they linked to contains many questions that really address extraversion (e.g. 1, 6, 7, 10). So non-narcissistic extraverts would score higher than introverts on that test. When they say that the most productive group was half-narcissists, this also means it was half-extraverts. Extraverts, compared to introverts, would be better on average in both those scenarios: at pitching ideas (appearing "more enthusiastic, witty, and charming") and generating ideas in groups. Because of this, it's hard to say what role narcissism played, separate from extraversion.