That's an Interesting point, Turnip.
On the subject of instruments, I’m wondering if the study includes the instrument of the voice. The voice uses similar skills to the piano or guitar, ie interpreting the notes on the page, their lengths and dynamics, etc, and translates this interpretation to the instrument with the aim of conveying emotion.
Not only that, singing encourages flexibility for the vocal chords (like the fingers with the piano or guitar) not to mention, of course, the immense amount of pleasure the performer – and maybe even the listener – enjoys!
Should a person with PD give up singing and/or playing? For me, the answer is a no-brainer!