Shorting Triac gate to ground doesn't turn it off
You have misunderstood the circuit description. S1 represents the "points" in a conventional ignition system, and it's the opening of the contacts that initiates a spark.

In any case, a triac cannot be turned off via its gate anyway. The only way to turn it off is to allow or force the current through the main terminals to fall below the device's holding current value.