help understanding this reverse polarity protection circuit
the -100V is a 5A inductive pulse, whereas DC survival spec is only -30V +50V.

The reason for this -30V is in case a alternator charged 24V vehicle is used to jumper cable another with the cables reversed. (momentarily)

The 2nd simple circuit is missing parts and will not survive all the test conditions for  ISO-16750-2 tests 1 thru 5b .  Keep this date in mind when reviewing solution documents.

For more details, consult with ISO-16750-2 or the older (2010) ISO7637-2 test levels on each pulse and Steady state conditions.  THere are different classes III and IV with pulse levels up to -600V for 24V system.