Pure speculation. OK? Pure speculation:
I would not be surprised if any fire that was started was because the bullet struck a hard rock in the backstop, not from any localized "flash" when it struck the gong.
Notice all the sparks when you see a video of a car dragging hunks of metal? And asphalt is not all that hard.
Think of a grindstone whizzing against a hunk of steel. Localized temps can be very high.
In addition, if iron gets hot enough, it will start to burn all on its own.
And it doesn't have to reach the ignition temperature from outside sources of heat. Get it warm enough, and the increased oxidation rate will accelerate the heat rise until it does get hot enough to burn. It is, in a word, mildy pyrophoric. (Like depleted uranium, but less so.)
Fires have started from iron cuttings, shavings, and chips getting wet. The oxidation rate keeps increasing until the iron is hot enough to self-ignote. The iron + oxygen reaction is very exothermic.
Welders know that once you start a cut with the oxy-acetylene flame, you can shut off the acetylene, and go on cutting just from the oxygen from the torch making the iron burn, releasing enough heat to continue the cut without any additional fuel (acetylene) required. At that point, the iron becomes its own fuel.
Also, up on the Grasslands in CO, there are many chunks of flint just lying around on the ground. I found one once that was about 2 pounds. I would guess hitting one of those with a steel (jacketed or cored) bullet might well throw a couple of sparks.
OK. Pure speculation. OK?
I suggest that anything moving along at >2000 fs striking something standing still will create very high temps. With lead and cupro-nickel, it wouldn't matter. But with steel, the iron in the small particles scattering might well ignite lighting up any tinder in the area.
Pure speculation, based on mechanical experience, not actual shooting experience, OK?
OK.