It doesn't matter much for this application if there is a big gap between the turn on and the turn off voltage.
Headlights take lots of current on turn-on, enough to operate a current relay even if the relay needs a lot more than 8.3 A to turn on.
When a bulb blows, it almost always drops to no current at all, so any relay will drop out.
If there is some reason that exactly 8.3 A is important, then a relay would be useless. However, the current in a 100 W 12 V bulb will fluctuate around 8.3 A depending on temperature, age of the bulb, and supply voltage. The voltage on a car battery varies a lot, so the current could be as low as 4 or 5A and the bulb could still be OK.