Under the ability 'Immunity to Magic', an iron golem is said to absorb fire damage and heal for an amount instead.
Immunity to Magic (Ex)
An iron golem is immune to spells or spell-like abilities that allow spell resistance. Certain spells and effects function differently against it, as noted below.
A magical attack that deals electricity damage slows an iron golem (as the slow spell) for 3 rounds, with no saving throw.
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. If the amount of healing would cause the golem to exceed its full normal hit points, it gains any excess as temporary hit points. An iron golem gets no saving throw against fire effects.
An iron golem is affected normally by rust attacks, such as those of a rust monster or a rusting grasp spell.
Mythic fireball bypasses resistance and immunity.
If you expend two uses of mythic power, the maximum damage increases to 20d10, the area increases to a 40-foot radius spread, and any fire damage dealt by the spell bypasses fire resistance and fire immunity.
So the question becomes thus: Is the absorb component considered an immunity, as it is listed as part of an immunity ability? In addition, does it block the mythic fireball high level mythic ability intended to let it punch through such things?
I would say no because of the way its worded. That is to me it reads as . . .
Statement: A golem is immune to spells or spell like abilities that allow spell resistance. End Statement.
New Statement: Certain spells and effects function differently against it . . . rusting grasp spell. End Statement.
The ability to heal when hit with a fireball is not an immunity or damage reduction its healing from the unique interaction of the fireball spell with an iron golem. Rather than being immune to fireball (since it grants SR) the iron golem instead heals from it so a fireball effectively bypasses its SR automatically like a PC voluntarily allowing a heal spell to affect them. If you just had the first statement (It is immune to spells that give spell resistance) then the mythic fireball would bypass that. However fireball specifically also heals and mythic does not bypass that statement.
The spell bypasses Fire Resistance. It doesn't bypass Spell Resistance. Fire Resistance lessens the damage you take from fire spells. Spell Resistance prevents the spell from ever effecting you. Channel Power will allow your spells to bypass Spell Resistance.
except it doesn't because it literally says it does not allow you to bypass immune to magic or immune to spell effects