It's common to say a car is powered by fuel. It's also common to say a car is powered by its engine. The latter is indeed is a device that brings fuel and air close together (and does some more important things). It's less common, but certainly not wrong, to say a car is powered by combustion.

Similarly, I think saying both 'the sun is powered by gravity', 'the sun is powered by hydrogen' and 'the sun is powered by fusion' is reasonable.