That's something that Dan said during his presentation. I don't know how he arrived at that result, but at least for the major blockchains, polls have been done in this area. This is from a quick google search which claims less than 2% of bitcoin users are female: https://cointelegraph.com/news/its-a-mans-world-only-176-of-bitcoin-community-are-women

I do remember seeing a post a while back that claimed more than 50% of Steemit users are female, not sure. Once again, this is Dan's claim, not mine, but I would guess it's probably true.