musicnews - no, I'm not wrong. Indeed, artist/genres is returning some data, but this is a non-documented and non-supported call and the data it returns should not be counted on to be correct, nor should you expect that method to always return data. To get the genre for an artist use the genre bucket like so:
http://developer.echonest.com/api/v4/artist/profile?api_key=YOUR_API_KEY&id=ARH6W4X1187B99274F&bucket=genre
The documentation in your screenshot is describing the artist bucket:
