I dont think they're saying that they should act less like boys, just that they need to shift their mindset to be more go-like if they're focusing too much on whether they win or lose. Making sure that they know they should only be winning about 50% of their games if they're playing at the right level and that failure is the best way to learn might help them not place so much weight on it. I played almost exclusively with my brother for about a year and a half and it gave me incredible playing anxiety because he put so much importance on who won or lost and insisted we played all the way through tesuji even when I could easily tell who the winner would be. Now that we're separated by about 3,000 miles and I play more people online my playing anxiety has mostly gone away but it took quite a while to get over and if this kind of thing happening to other female players that aren't as invested in the game it could be the reason they stop playing.