It's not even against all 'physical damage'. Falling damage, for instance, ignores it, along with oh so many other things.
A creature with this special quality ignores damage from most weapons and natural attacks. Wounds heal immediately, or the weapon bounces off harmlessly (in either case, the opponent knows the attack was ineffective). The creature takes normal damage from energy attacks (even nonmagical ones), spells, spell-like abilities, and supernatural abilities."