The number of exceptional people: Fewer than 85 per 1M across key traits
sciencedirect.com
sciencedirect.com
The paper is simply computationally determining the combined frequency. My guess is that somebody simply needed a citation for this calculation.
Calculating the exact number looks like a nightmare. I'd try with Wolfram Alpha and hope it can integrate it numerically. Otherwise, I'd use the Montecarlo method, that is equivalent to their method, but they use "N = 20 million" that is pretty small, I think with a x1000 the calculation still takes less than a second and the error would be like 30 smaller.