This particular engine was never standard but I've flown other IO-375s with standard fuel injection. As you say, shutting one down with ignition rather than fuel is normally a non-event as the fuel mixture becomes initially way too rich and then way too lean followed by cutoff as the RPM decreases. It's possible to get a combustion event or two during the shutdown but normally everything just winds down peacefully. On this fuel system everything is completely replaced from the tank pickups all the way to the injector ports on the cylinders. Fuel pressure is provided by a primary fuel pump with a backup fuel pump in parallel that kicks in if the power to the primary fails or if the fuel pressure drops below 20 psi. It doesn't matter what the fuel pressure is though, the injectors will only deliver fuel on the intake stroke.
During an ignition only shutdown the fuel system is still working at 100% all the way through the shutdown. As the MAP increases and RPM decreases the system is adjusting for this and metering the fuel accordingly so it's much more likely on any given power stroke for a combustion event to happen. This is why the system is normally shut down with the key to kill everything at once, injectors and ignition.