> For example, learning to play a piano is training the spine, not the brain. Because the feedback loop with the spine is shorter.

It may train the spine too but good luck trying to play the piano without a brain. More concretely, your eyes read the notes you like to play, your ears provide the feedback loop whether you are playing the right thing (and dynamics and interplay with other musicians in an ensemble), and you need things like planning ahead, sense of form, etc. Twiching fingers from some spine impulse is just part of playing the piano. (Besides, the latency between brain and finger muscels is insubstantial w.r.t. the time scales you need for general music playing, beyond certain nuances.)