Oh, seems like Apple has renamed the setting in iOS11. If I remember correctly it has been called "Media Library" or "iTunes Library". Anyway, if you disable this setting, Stage Traxx will no longer be able to access any music from your iTunes library regardless wether you actually use Apple Music or not. So the wording is quite misleading in this dialog. But unfortunately I cannot change the wording there.
If you actually use Apple Music, all songs downloaded from Apple Music will have DRM embedded. Only songs that you have purchased will be downloaded without DRM. 3rd party apps like Stage Traxx that do more than only start and stop playback, have no way to access DRM protected music. Therefore it is best practice to disable Apple Music on the device where you run Stage Traxx.
The problems described in this thread will only occur with local files, not iTunes files. So if you switched back, to iTunes files, you will not see the specific error described here.
