"...was Chris allowed to go off the line ahead of Jeff (get a green light first)..."
Yes.
"...was it his reaction time that got him the win? "
A good RT helps. You also need to run close to your dial-in, without going quicker. If both cars go quicker, then the car that runs closer to it's dial-in wins. You can dial-in any ET you want as long as it's not slower than the NHRA index for your class.
If 2 cars in the same class run, it's a heads-up race. 1st car to the finish line, without a red light start, wins.
