To say something has less impact than something else is not to say it has none.

She is saying that once women have a real choice, and are not pressured into going for the most economically rewarding, they will choose fields they like rather than those. Today, women in more economically prosperous countries, and with more opportunities, have a choice, and more will choose the fields they prefer, i.e.teaching, social work, medicine, law, accounting, finance, biology, etc., but not C.S. 

Part of it is because they are less driven by economic and status considerations, and more by intrinsic interest, part of it because they don't want the lifestyle that comes with the environment that is so much more stressful in the field (i.e. the environment) than it was 30 years ago.