If you're talking about a person using an LLM, or some other ML system, to help generate their music then the LLM is really just a tool for that person.
I can't run 80 mph but I can drive a car that fast, its my tool to get the job done. Should I not be allowed to do that professionally if I'm not actually the one achieving that speed or carrying capacity?
Personally my concerns with LLMs are more related to the unintended consequences and all the unknowns in play given that we don't really know how they work and aren't spending much effort solving interoperability. If they only ever end up being a tool, that seems a lot more in line with previous technological advancements.