We might get around the dilemma by admitting Yoko and Rotten do NOT make music, therefore, they are NOT musicians.
Vocal chords are instruments. Playing the vocal chords well takes as much skill, talent, and years of practice as any manmade instrument.
We have the same semantic problem with the words, artist, painter, photographer. Does that mean anyone who has drawn a stick figure, finger painted in kindergarten, or pushed a shutter release is an artist, or painter, or photographer?