I think I understand this now. They are using the body diode in the mosfet, not the mosfet itself. The gate is tied to the source to keep the mosfet turned off. And since the mosfet is hooked up in reverse, you are actually using the body diode as your reverse polarity protection.
So this assumes the mosfet body diode is better than a standard rectifier?
You would not need any gate protection for this, since the gate is tied to the source, it will always be safe from transients. Zener diodes are only to protect against transients, not a full time over-voltage condition.