If they're covert and only members of the group using them know what they mean, then why fight it? Seems like a helpful way to communicate about things that others might not like to hear. The paper you linked said that too - "Such signals may allow coordination and enhanced cooperation while also avoiding the alienation or hostile reactions of individuals with different preferences.".

This happens all the time in cartoons that appeal to children but also contain subtle adult jokes so everyone can enjoy them on different levels.