I am currently attending college, and I can confirm that many at my age refer to a good girl friend as a "bro". It may be stupid and immature, but it is what it is. As other commenters has said, "bro" is describing the relationship and not necessarily referring to any specific gender.

Being a "bro" is an attitude thing. The term is stupid, the ad is stupid, but it's not sexist.

Calling anybody "bro" is rarely serious. I would actually feel less stupid calling a woman "bro" than a man, because if I call a guy "bro" people might think I was serious.