but the problem is, other than 'princess' dresses (and I suppose maybe the very old fashioned white-apron-with-red-cross 'nurses' outfit, which is well out of date now!) any other fancy dress costume is, actually gender neutral, as it basically consists of accessories related to a specific job, which can be performed by someone of any sex. So a stethoscope and white jacket worn by a girl or a boy would be a doctor, same with a fake gun, checked shirt and lassoo would be a cowboy/cowgirl,etc. etc.
But what they are saying is that a girl wearing scrubs and a stethoscope, which she takes from the blue box marked 'boys dressing up clothes,' isn't 'A girl dressing up as a doctor,' it's 'a girl dressing up in a boys costume.' which becomes in a 4-year-old's mind 'doctors are boys, girls can't be doctors.'
It depends...if it's just clumsy wording by the school it's an easy mistake. If, however, it's indicative a) of what will happen to the costumes once they are donated ("No, Ellie, you can't dress up in that, that's for the boys, why don't you go and put the Elsa dress on?") and b) the internal thought process of the teachers and wider school, it is something to pick up on, because it can lead to wider, more insidious,knock on effects, like a pp mentioned, for example a little girl being told off for being boisterous when a boy would be handwaved for doing the same thing, or a little boy made to play football when he wants to play hairdressers, and so on.