Yes you can. Just say, "an oscillating circuit where you control the frequency and amplitude with your hands, sculpting sound in the process, is the epitome of modulation". Or write it on the blackboard 50 times, as you have to stay after class here at Synth Skool.
Tubes and circuits don't count? If so, why not?
I don't consider pitch and amplitude exactly sculpting an original sound generator output as that's what the generator should do, provide pitch and amplitude levels. What happens after that is synthesis. Before that it is a sound generator, nothing else.
The tube circuits case it where you could use some kind of legal means to try to make a theremin a synthesizer in a court case, indeed.