I don't know if they still do this. I haven't seen a physical dictionary in a while. Now with online maps and navigation I don't think map data providers can afford the risk of a navigation system misdirecting someone due to an imaginary place.
But it's possible some of these things took a life of their own. Say someone saw a made up word and started using it because they thought it was real. Or a made up park name for an open area led someone to start using it and others to start calling it that. I don't know if that actually happened but it's possible.