It is not at all like playing an instrument.
Instruments present a clear interface to a user, have predictable outputs, etc.
The only comparison that might work for me is that LLMs are very bad instruments where you are constantly forced to negotiate its idiosyncrasies in order to massage the output you want from it, and even then there is enough randomness that trying to do so is almost a fool's errand.
I think they mean playing different instruments not other instances of the same instrument. A tuba's interface differs from a violin's, etc.
My criticism of the comparison would stand in either case. There is nothing clear and uniform about the interface to LLMs that match their musical counterparts. Even modular synthesizers with random sources are far more controlled.
I also think it's disingenuous to call LLMs "tools" in the stricter sense of the definition, but I've mostly given up trying to convince people of this. Main reason being that a terrible writer and a gifted writer can produce similar outputs, and for the terrible writer it will be above their average, and for the gifted writer it will be below what they could produce with full control.