But there is a 200 ohm resistor in between, that reduces current to (12-5) / 200 = 35mA.
Hopefully to less than what the Arduino itself consumes (The Uno draws 50mA).
If fault current could be more, then a 5volt (standoff) TVS diode (super-zener) across the 5volt supply of the Arduino would be more sensible than a 5volt zener from pin to ground.
Leo..