I think this article is full of shit. No way it's even close. My guess is about 20 % of that number.
I know there are about 170 Black belts in Wisconsin (we keep a list).
Wisconsin Population is 5.9 million
Therefore, 1 in 34,705 people in Wisconsin is a black belt (Assuming Wisconsin is an average BJJ density). (or .000028 percent)
In the United States, using the .000028 figure would result in approximately 9,300 black belts in the country. Assuming a higher density in some states, let's go with an even 10,000 in the US. Maybe squint and get to 11,500.
The US has to have the second highest number of BJJ Black belts, behind Brazil. If Brazil has twice the occurrence rate of black belts as the US (.000056), they have approximately 12,500.
After that, you are talking countries that didn't even start getting black belts until the mid or late 2000's. I found an article from 2015 that put the total number of blackbelts in Europe at about 1,000. Let's say that's gone up 7X in the last 10 years. You are still talking about only 7,000 blackbelts in Europe.
The only other real hotbed internationally would be Japan. Maybe we add another 500 - 1000 there. Add another 1000 for Australia, New Zealand, and really anywhere else in the world.
Now, we are looking at -
Brazil - 12,500
US - 11,500
Europe - 7,000
Japan - 1,000
Everywhere else - 1,000
Total = 33,000 BJJ black belts
I'd love to see their methodology to get to a number 5 times higher than that.