With the recent update, Spotify changed the artist tab to only show artists you are following, and recommended artists underneath that. So, the artists tab no longer shows all the artists present in your library, only ones that you have specifically followed.
Yes! It's only for desktop. I just used it myself and it worked perfectly. You might have to restart the app after using this to have your artists show up. It should then sync to your phone (I had to restart my phone app, too).