The difference is that a violin player can predict how the known violin will behave under all relevant circumstances, will know how to get the right tone out of it, while you’re generally unable to predict the adequacy of output of even a deterministic LLM. You can’t practically reason about how varying the input to the LLM will ensure the adequacy of its output, while the violin player is perfectly able to do so for the violin.

This is because LLMs have aspects of chaotic dynamical systems, where small changes in initial conditions can lead to vastly different outcomes. That property is independent from nondeterminism.