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I think describing the switch as a rev limiter is more accurate than describing it as a runaway preventer.
When a certain vacuum is reached, at high RPM, the switch closes grounding out the set of points it’s hooked up to. This kills the spark on that cylinder and the motor slows down. In an outboard motor runaway situation, the motor revs up on its own uncontrollably because of preignition and no spark is needed to keep it running. The preignition is usually caused by an air leak which leans out the mixture and raises the combustion chamber temperatures. As a result, the spark plug or a piece of carbon will become red hot which will ignite the fuel charge before its time. Turning the key off or pulling the spark plug wires off will not stop the motor as by now the ignition system is redundant and just there for the ride. Flooding the motor by choking it to death will bring it to a stop.
A runaway diesel motor is a whole different ball game. When they runaway, turning off the fuel supply has no effect as they are usually running on their own crankcase oil. A blown oil seal in a turbo will cause this and the motor will keep screaming away until it either breaks up or runs out of lube oil. Either way, it’s not good for the motor. Yeah, I know, strangling its air supply will shut it down to but diesels usually have a robust air filter system and by the time it’s removed to gain access to the intake, the damage will have been done.