I don't believe that bullets hitting rocks will start a fire, but the Air Force has loads of video that says your claim about tracer not igniting gas is BS. I, and many other Army and Marine Vets on this site have spent PLENTY of time putting out grass fires ignited by tracer.
I have personally seen one discarded cigarette start a brush fire in a somewhat dry (no recent rain) but moist (Spring) setting.
Let's talk drought. Having lived through drought conditions here in ND, I have seen the result of a combine hitting a rock and and the resulting small spark starting an 800 acre fire. Drought conditions are not normal, and they create absolute tinderboxes. It got so every morning when I got up on my ranch I scanned the horizon in 360 degrees, and didn't stop until I went to bed or it got dark.
Conditions have to be right - the combine alone won't start a fire, for example, (although a hot muffler on a pickup will) the rock has to be in the right place to hit the swather head, and the tinder needs to be right there. But in a hard drought, as in northern CA right now, it is not all that hard. The tree huggers also made it just that much easier, and that much more likely.