Exactly...the backwards current through the zener flows when the voltage is 5.1V or higher, but the zener voltage stays (relatively) constant at 5.1V, no matter how much current flows (until SO much current flows that it fails).
So when your input voltage goes from 14.4V to 20V, 30V, 40V, etc. the zener will stay at 5.1V and just conduct more and more current flowing (because the input voltage is getting higher). It's better that this current goes through the zener than into the Arduino pin!
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