Before the list of software contractor was smaller, so the few places you could work in was NASA and Military.

Now the variance of jobs is much greater, so not all jobs are NASA level hard.

Who thinks that? Hard is something relative. The author says that men could be more prepared to tech because a biological predisposition that makes tech easier positions for them (better abstract thinking and more tolerance for stress, for example), while women are more prepared for other things likes taking care of others (because they are better in socializing and empathy). As tech became more competitive the women just left it for other things while men get into to get more status.