I see your line of reasoning.
But I think the wording of the ability:
'In addition, certain spells and effects function differently against the creature, as noted below.
A magical attack that deals fire damage breaks any slow effect on the golem and heals 1 point of damage for each 3 points of damage the attack would otherwise deal.'
should be read (literally) to indicate that the magical attack itself is changed into healing damage.
Otherwise wouldn't it read:
When the golem is subject to a magical attack, the golem heals 1 point of damage for each 3 points of damage the attack would otherwise deal?
Contrast:
For each such negative level bestowed, the wight gains 5 temporary hit points.
with
A magical attack that deals fire damage . . . heals 1 point of damage
But I think the wording of the ability:
'In addition, certain spells and effects function differently against the creature, as noted below.
A magical attack that deals fire damage breaks any slow effect on the golem and heals 1 point of damage for each 3 points of damage the attack would otherwise deal.'
should be read (literally) to indicate that the magical attack itself is changed into healing damage.
Otherwise wouldn't it read:
When the golem is subject to a magical attack, the golem heals 1 point of damage for each 3 points of damage the attack would otherwise deal?
Contrast:
For each such negative level bestowed, the wight gains 5 temporary hit points.
with
A magical attack that deals fire damage . . . heals 1 point of damage