I’ve taken to interpreting “girl+noun” to be a state of being, and a adjective itself; used by girls, women, or feminine identifying individuals.

Another example with be “girl dinner” which is a when a feminine identifying individual is eating a dinner they find to be silly or funny in some way.

The author is implying a message to empower women to unionize. Not including men in a message does not mean men can’t or shouldn’t unionize.