It does not surprise me in the least that a girl would feel comfortable- or even more comfortable- practicing with the boys. I cannot speak for Aussie coach but what I have observed is that some (many?) boys feel uncomfortable working gymnastics with groups of girls. An individual or two here and there of a different sex is probably not going to matter, especially if they are very young. It is more a question of critical mass.
Gymnastics is a tricky sport for boys in a whole host of ways. It is hardly considered a super masculine sport. (Obviously this is absurd, but let's face it, it is true as far as how the sport is perceived in the greater culture.) This can be quite difficult for some boys. Being on an all boy team helps IMO.
Also, I think that even a girl who hated WAG, refused to do it, thought it was stupid, and only wanted to do MAG (or football or wrestling or whatever) could still identify entirely as a girl! I think there is far too much stock being placed in this oppressive idea that we are somehow not able to identify entirely as one "gender" if we do not adhere to some strict stereotyped straightjacketed version of that gender! No. Women can be very "fem" and very "butch" and most are something in between. And sometimes both in the same lifetime. (Or even the same day.) And the same is true of men. I don't know. This was clear way back in the 70s & 80's when I was growing up and most of the boys and girls dressed basically the same! I do not know what has happened but it is very sad that kids are made to feel somehow not entirely their own sex because they prefer activities or dress or behavior someone has degreed as belonging to the opposite sex. Who the heck is making these new rules? I find them very damaging.