Yes, you could in theory call a singer an instrumentalist, there'd be nothing orthologically or semantically incorrect with that.

But we also have to accept common usage if we wish to make ourselves understood.

Another example ... a bus is powered by a motor. So if I caught the bus to Brighton, I could quite correctly say, "I motored down to the coast".

But if I did say that, you'd assume that I'd driven there in a car; not because my statement was factually incorrect, but because, in everyday speech, the verb 'to motor' has become synonymous with 'to drive a car'.