Resistant skin doesn't reduce spell damage, hero armor does.
Resistant skin makes spells cast on the unit with this ability to behave as if cast on hero unit.
For example Bash ability. It deals 25 bonus damage and stuns normal units for 2 seconds and hero units for 1 second.
A normal unit with resistant skin will take 25 bonus damage and will be stunned for 1 second just like heroes get.
Resistant skin will also prevent abilities like Polymorph to be cast on the unit, because that ability can be cast on non-hero units only. Unit with resistant skin is considered a hero unit for spell effects.
