It bothers me too. The article is otherwise great.
google, define: girl "1. a female child. 2. a young or relatively young woman."
https://en.wikipedia.org/wiki/Girl: "A girl is any female human from birth through childhood and adolescence to attainment of adulthood when she becomes a woman."
http://www.merriam-webster.com/dictionary/girl: "a : a female child from birth to adulthood
...
d : sometimes offensive : a single or married woman of any age"
In this case, it seems like she is trying to sound cute or informal while unwittingly reinforcing the perception of women as child-like.