He doesn't assert the differences are due to inferiority. The vast majority of differences he mentions are differences of taste not differences of ability. There are many studies that show that women are more prevalent in engineering in poor countries than in rich ones. One fairly prevalent hypothesis to explain this data is that economic freedom allows people to pursue what makes them happier, rather than what makes them the most money. I have no idea if this hypothesis is right, but I think it's pretty sad that one can't make a case for it without getting fired.