High school boys in tech are much more likely to be misogynistic and socially inept than other high schoolers are. They make girls uncomfortable (they make me uncomfortable too, and I'm not event the subject of their creepiness), and this discourages girls from trying to program or build things.
There are women who can handle this quite well (the author of the piece is an example), but there are also many women who don't deal with this so easily. Those who can deal with the male-dominated environment will always be free to join mixed-gender events and groups, but girls' robotics classes target those who could gain a lot from having friends of the same gender who share similar interests.
Women aren't self-selecting themselves into tech, but there's no reason they couldn't. Women-in-tech events aren't fighting sexism with sexism, they're providing counterexamples to the misconception that programming is only for men.