The idea is that you're looking for the effect caused by gender/sex, so you have to remove the contributions of other factors. Theoretically, what the comment

The idea is that you're looking for the effect caused by gender/sex, so you have to remove the contributions of other factors. Theoretically, what the comment is saying is that women get paid less because they take more leave, not because they are women. Again, theoretically, if men took leave to the same extent, their salaries would also decrease to the same extent. That's the point of controlling for other variables, so that you can tease apart the individual effects they have before they're all muddled together.